Go: package main

import (
        "fmt"
        "bufio"
        "os"
)

func main() {
        var n int
        var a, b string
        sc := bufio.NewScanner(os.Stdin)
        fmt.Scanf("%d", &n)
        for i := 0; i < n; i++ {
                sc.Scan()
                a = sc.Text()
                sc.Scan()
                b = sc.Text()

                if len(a) > len(b) {
                        s := add(a, b)
                        if s[len(a)] > 0 {
                                if len(a) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(a)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                } else {
                        s := add(b, a)
                        if s[len(b)] > 0 {
                                if len(b) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(b)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                }
        }
}

// expects ln > lm
func add(n string, m string) (s [100]int) {
        ln := len(n)
        lm := len(m)
        var sum [100]int

        for i := 0; i < ln; i++ {
                var s, t int
                s = int(n[ln-1-i]) - int('0')

                if lm - 1 - i >= 0 {
                        t = int(m[lm-1-i]) - int('0')
                } else {
                        t = 0
                }

                sum[i+1] = (s + t + sum[i]) / 10
                sum[i] = (s + t + sum[i]) % 10
        }

        return sum
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <algorithm>

using namespace std;

int main (void) {
    int N;
    cin >> N;
    while (N--) {
        string s1, s2;
        cin >> s1;
        cin >> s2;
        vector<int> n1, n2, res;
        for (int i = s1.size(); i--;) {
            n1.push_back((int)(s1[i] - '0'));
        }
        for (int i = s2.size(); i--;) {
            n2.push_back((int)(s2[i] - '0'));
        }
        if (n1.size() < n2.size()) {
            for (int i = n1.size(); i < n2.size(); i++) {
                n1.push_back(0);
            }
        } else if (n2.size() < n1.size()) {
            for (int i = n2.size(); i < n1.size(); i++) {
                n2.push_back(0);
            }
        }
        int s, c = 0;
        for (int i = 0; i < n1.size(); i++) {
            s = n1[i] + n2[i] + c;
            res.push_back(s % 10);
            c = s / 10;
        }
        while (c != 0) {
            res.push_back(c % 10);
            c /= 10;
        }
        int size = res.size();
        while (size > 0 && res[size - 1] == 0) {
            size--;
        }
        if (size > 80) {
            cout << "overflow" << endl;
        } else {
            int i = res.size();
            while (i--) {
                cout << res[i];
            }
            cout << endl;
        }
    }
    return 0;
}

C++: #include <iostream>
#include <string>
#include <cstdlib>
#include <algorithm>

#define MAX_DIGIT 80
#define ERROR ("Overflow")



int main(void){
	using namespace std;

	int N;
	cin >> N;

	vector< pair<string,string> > input;
	input.reserve(N);

	for(int i=1; i<=N; i++){

		string slhs, srhs;
		cin >> slhs >> srhs;

		reverse(slhs.begin(), slhs.end());
		reverse(srhs.begin(), srhs.end());

		input.push_back(make_pair(slhs,srhs));
	}

	for(auto p=input.begin(); p!=input.end(); p++){
		auto slhs = p->first, srhs = p->second;

		if(slhs.size()>MAX_DIGIT || srhs.size()>MAX_DIGIT){
			cout << ERROR << endl;
			continue;
		}

		int lhs[MAX_DIGIT], rhs[MAX_DIGIT], res[MAX_DIGIT];
		fill(lhs, lhs+MAX_DIGIT, 0);
		fill(rhs, rhs+MAX_DIGIT, 0);
		fill(res, res+MAX_DIGIT, 0);
		auto atoi_digit = [] (char x) -> int {
			return atoi(&x);
		};
		transform(slhs.begin(), slhs.end(), lhs, atoi_digit);
		transform(srhs.begin(), srhs.end(), rhs, atoi_digit);

		int headDigit;
		for (headDigit=MAX_DIGIT-1; headDigit>=0; headDigit--){
			if(lhs[headDigit]!=0 || rhs[headDigit]!=0){
				break;
			}
		}
		if(headDigit != MAX_DIGIT-1){
			headDigit++;
		}

		int carry = 0;
		for (int j=0; j<=headDigit; j++){
			auto d = div(lhs[j]+rhs[j]+carry,10);
			carry = d.quot;
			res[j] = d.rem;
		}
		if(headDigit==MAX_DIGIT-1 && carry !=0){
			cout << ERROR << endl;
			continue;
		}


		for (; headDigit>=1; headDigit--){
			if(res[headDigit]!=0){
				break;
			}
		}
		for (int j=headDigit; j>=0; j--){
			cout << res[j];
		}
		cout << endl;
	}
}