ModAdd

#include <iostream>
#include <string>
#include <algorithm>
using namespace std;

int main() {
    std::string a, b;
    cin >> a; cin >> b;
    if (a.length() < b.length()) std::swap(a, b);

    int n = a.length();
    int m = b.length();
    bool leadingZeros = true;

    for (int i=0; i<n-m; ++i) {
    	leadingZeros = false;
    	cout << a[i];
    }
    for (int i=0; i<m; ++i) {
    	int res = ((int)(a[n-m + i]) - 48 + (int)(b[i]) - 48) % 10;
    	if (res != 0 || !leadingZeros) {
    		cout << res;
    		leadingZeros = false;
    	}
    }
    if (leadingZeros) cout << "0";
    cout << "\n";
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED