Reading a Problem's Input

cin >> x reads ONE whitespace-separated piece from the input and puts it in x. A space and a new line are the same separator to cin, so where a line ends makes no difference to it.

int a, b;
cin >> a >> b;
cout << "a=" << a << " b=" << b << endl;

(input: 3 4)
a=3 b=4

That program prints exactly a=3 b=4 whether the input arrives as one line 3 4 or as two lines, 3 then 4. That is why the shape of the input rarely troubles you in C++.

C++ does not have this problem. cin >> treats a space, a tab and a new line all the same way — as a separator to skip over. So one line saying 3 4 and two lines saying 3 and 4 look identical to cin >> a >> b;.

int a, b;
cin >> a >> b;      // one line "3 4", or two lines — both work
cout << a + b << endl;

(input: 3 4)
7

The COUNT still matters in C++ though. If the format promises two values and you write cin >> a >> b >> c;, the third value never arrives: cin fails, c keeps whatever junk was in it, and what you print is a meaningless number.

Careful the other way: when the input really is two lines with one number each, writing int(input()) twice is CORRECT. It is only wrong when both numbers sit on one line. Do not over-correct and start cutting up lines that hold a single number.

In C++ you never cut the line yourself — cin sees the input as a stream of whitespace-separated pieces, and each >> pulls one piece off. It is still worth knowing what those pieces are.

int a, b, c;
cin >> a >> b >> c;
cout << a << "|" << b << "|" << c << endl;

(input: 7 2 9)
7|2|9

string a, b;
cin >> a >> b;
cout << a + b << endl;

(input: 3 4)
34

In C++ the conversion is decided by the variable's type. Declare int a; and cin >> a turns the piece it read into a number for you. Declare string a; and it keeps it as text.

int a, b;
cin >> a >> b;
cout << a + b << endl;

(input: 3 4)
7

Everything an Input format paragraph can say is really one of a handful of shapes. Learn them once and after that you only have to recognise them.

int n;
cin >> n;
cout << n * 2 << endl;

(input: 12)
24

int a, b, c;
cin >> a >> b >> c;
cout << a + b + c << endl;

(input: 7 2 9)
18

double x;
cin >> x;
cout << x * 2 << endl;

(input: 1.5)
3

Never print a prompt like "Enter a number:". The judge compares your output character by character, so any extra text turns a correct answer into a wrong one.

cin >> name stops at the first space, so from "Bat Erdene" you only get "Bat". When you need the whole line, right to its end, use getline(cin, name).

string name;
getline(cin, name);
cout << "Hello, " << name << endl;

(input: Bat Erdene)
Hello, Bat Erdene

string name;
cin >> name;
cout << "Hello, " << name << endl;

(input: Bat Erdene)
Hello, Bat

Writing getline(cin, s) straight after cin >> n leaves s EMPTY: the Enter that followed the number is still sitting in the input, and getline reads that leftover instead. The getline lesson deals with this properly.

In C++ cin throws nothing — it fails quietly. If you read with >> after the input has run out, the variable is left with no value and EVERY later read stops working. The program does not crash, so the judge says "Wrong Answer" rather than "Runtime Error", which makes the cause harder to find.

int a, b, c;
cin >> a >> b >> c;   // ✗ оролтод ердөө хоёр тоо байхад гурав уншив
cout << a + b + c << endl;

The number of >> reads must match the number of values in the input format exactly. If you are unsure, print what you read and look at it.

This section uses loops and lists, which come later. You do not have to learn them now. You will want to copy from here while solving problems, though, so here they are.

Line 1 holds HOW MANY numbers there are, then come the numbers themselves. Read n, then run a loop n times, reading one number each time round.

int n;
cin >> n;
int total = 0;
for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    total += x;
}
cout << total << endl;

(input: 3\n1 2 3)
6

That loop works for both layouts: it prints 6 whether 1 2 3 arrives on one line or on three separate lines. This is what cin ignoring line breaks buys you.

int n;
cin >> n;
int total = 0;
for (int i = 0; i < n; i++) {
    int x;
    cin >> x;
    total += x;     // ЯГ ижил код — cin мөрийг үл хайхарна
}
cout << total << endl;

(input: 3\n1\n2\n3)
6

int n;
cin >> n;
for (int i = 0; i < n; i++) {
    int a, b;
    cin >> a >> b;
    cout << a + b << endl;
}

(input: 2\n1 2\n10 20)
3
30

On every new problem, do the same four things to the Input format paragraph. After a few problems it becomes automatic.

1. 1.Count how many LINES the input has.
2. 2.Count how many VALUES sit on each line.
3. 3.Write the matching line of code for each line of input.
4. 4.Add no prompt and no extra text to your output.

In C++ step 3 is even simpler: one >> per value, in the order the format lists them. You need not count where the lines end — only the values.

int n;
cin >> n;
int a, b;
cin >> a >> b;
cout << "n = " << n << endl;
cout << "a = " << a << endl;
cout << "b = " << b << endl;

(input: 5\n3 4)
n = 5
a = 3
b = 4

Delete those printing lines once you are satisfied. On the judge they are extra output, and they will turn a correct answer into a wrong one.