Beginner · I/O basics

Sum of Two Numbers

Read two integers from input and print their sum. The "Hello World" of problem solving — the point is to master the read-compute-print cycle.

At a glanceRead two integers from one line and print a + b — the smallest complete program here: read, add, print, with negatives as the only edge case.

Problem

Input: one line with two integers a and b, separated by a space (−10⁹ ≤ a, b ≤ 10⁹).

Output: a single line with a + b.

Sample:

InputOutput
3 47

Your solution

main.cpp — your solution
Custom input for "Run my input"

Judging runs in your browser against hidden tests. A runaway loop is cut off after 3 seconds — still, make sure every loop has an exit. Your code is saved in this browser as you type.

Stuck? Hints

Hint 1 — the reading part

cin >> a >> b; reads both numbers in one line — >> skips the space between them automatically.

Hint 2 — the printing part

cout << a + b << endl; — you can compute inside the output statement, no extra variable needed.

Solution (try it first!) 
#include <iostream>
using namespace std;

int main() {
    int a, b;
    cin >> a >> b;
    cout << a + b << endl;
    return 0;
}

The whole problem is one cycle of the main pattern: read → compute → print. Next, make it a loop problem.

Deep dive

Why this works

The whole program is one pipeline: read, compute, print. Every harder problem on this site is this same pipeline with a loop inserted in the middle — learn to see the three parts and the rest is detail.

The cost

O(1) time, O(1) space. Two ints in, one out; nothing scales with input size.

Where it goes wrong

  • Reading with cin >> a >> b in the wrong order still compiles and still prints a number — just the wrong one. Trace values, not syntax.
  • Two billion plus two billion overflows int to a negative number. The test data here is small, but long long is the habit that survives.

Next step

The same pipeline with a loop in the middle: Sum from 1 to N.

Two things to remember

  • Every program is read → compute → print. Name the three parts before writing code.
  • Overflow is silent: int stops at 2,147,483,647 and wraps around.
🐞 This version fails — open it and try

Prints sum = 7 — the judge wants the bare number

#include <iostream>
using namespace std;

int main() {
    int a, b;
    cin >> a >> b;
    cout << "sum = " << a + b << endl;
    return 0;
}