ModAdd

#include <iostream>
#include <string>

int main()
{
    std::string a, b;
    std::getline(std::cin, a);
    std::getline(std::cin, b);

    auto ita = a.rbegin();
    auto itb = b.rbegin();

    std::string q;
    while (true) {
        if (ita == a.rend() && itb == b.rend()) break;

        int va = 0;
        if (ita != a.rend()) {
            va = *ita - '0';
            ++ita;
        }

        int vb = 0;
        if (itb != b.rend()) {
            vb = *itb - '0';
            ++itb;
        }

        q += std::to_string((va + vb) % 10);
    }

    auto r = std::string(q.rbegin(), q.rend());
    auto i = r.find_first_not_of('0');

    if (i == std::string::npos) std::cout << "0" << std::endl;
    else std::cout << r.substr(i) << std::endl;
}

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