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>
using namespace std;

int main() {
   int n;
   string str_a;
   string str_b;
   string sum = "";

   scanf("%d", &n);
   for(int i=0; i < n; ++i){
       cin >> str_a >> str_b;

   int id_a = str_a.size();
   int id_b = str_b.size();
   int c = 0;

   // ???????????????????????°?????????????????????
   while (id_a > 0 || id_b > 0) {
     int a = 0, b = 0;
     // int??§??°???????????????
     if (id_a > 0) a = str_a[--id_a] - '0';
     if (id_b > 0) b = str_b[--id_b] - '0';
     // ?????¨???????????°???????????????c?????????????????????????????°???????????????
     sum = to_string(((a + b + c) % 10)) + sum;
     // ?¬???????????????°????????????????¨????
     c = (a + b + c) / 10;
   }
   if (c > 0) sum = to_string(c) + sum;
   cout << sum << endl;
   sum = "";
   }
}

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

using namespace std;

inline int toInt(string s) { int v; istringstream sin(s); sin>>v; return v; }
template<class T> inline string toString(T x){ ostringstream sout; sout<<x; return sout.str(); }

#define dump(x) cerr<<#x<<" = "<<(x)<<endl;
#define debug(x) cerr<<#x<<" = "<<(x)<<"(L"<<__LINE__<<")"<<" "<<__FILE__<<endl;

int main(){
	int N; cin >> N;
	for (int i = 0; i < N; i++) {
		string a, b; cin >> a >> b;
		if (a.size() < b.size()) {
			string tmp("");
			for (int j = 0; j < b.size()-a.size(); j++) { tmp += "0"; }
			a = tmp + a;
		} else if (b.size() < a.size()) {
			string tmp("");
			for (int j = 0; j < a.size()-b.size(); j++) { tmp += "0"; }
			b = tmp + b;
		}
		
		string res("");
		for (int j = 0; j < a.size()+1; j++) res += "0";
		a = "0"+a; b = "0"+b;
		for (int j = res.size()-1; j >= 0; j--) {
			int cur = 0;
			if (res[j] >= '0' && res[j] <= '9') {
				cur = res[j]-'0';
			}
			int ai = a[j]-'0', bi = b[j]-'0';
			int res_tmp = ai+bi+cur;
			if (res_tmp >= 10) { 
				res[j] = '0'+(res_tmp%10);
				res[j-1] = '0'+(res_tmp/10); 
			} else {
				res[j] = '0'+res_tmp;
			}
		}
		int j = 0;
		while (res[j] == '0') { j++; }
		string ans("");
		for (; j < res.size(); j++) ans += res[j];
		
		if (ans.size() > 80) {
			cout << "overflow" << endl;
		} else {
			cout << ans << endl;
		}
	}

	return 0;
}

C++: #include <cstdio>
#include <cstring>
#include <algorithm>
#include <string>
#include <cmath>
#include <stack>
#include <vector>


using namespace std;

int main()
{
	int n;
	scanf("%d", &n);
	for (int i = 0; i < n; ++i)
	{
		char a[81], b[81];
		scanf(" %s %s", a, b);
		int aa[80], bb[80];
		memset(aa, 0, sizeof(aa));
		memset(bb, 0, sizeof(bb));
		for (int j = 0; j < strlen(a); ++j)
			aa[j] = a[strlen(a)-j-1] - '0';
		for (int j = 0; j < strlen(b); ++j)
			bb[j] = b[strlen(b)-j-1] - '0';

		int sum[81];
		memset(sum, 0, sizeof(sum));
		for (int j = 0; j < 80; ++j)
		{
			sum[j] += aa[j]+bb[j];
			sum[j+1] = sum[j]/10;
			sum[j] %= 10;
		}
		if (sum[80])
			puts("overflow");
		else
		{
			int d;
			for (d = 80; sum[d] == 0; --d)
				;
			for (; d >= 0; --d)
				printf("%d", sum[d]);
			printf("\n");
		}
	}

	return 0;
}