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 <cstdio>
#include <string>
using namespace std;

int main(void)
{
    int n;
    bool flag;
    scanf("%d", &n);
    
    while (n != 0){
        flag = true;
        
        int num1[81] = {0};
        int num2[81] = {0};
        int max_len;
        
        char in[3000];
        scanf("%s", in);
        // êUlðæèÞ
        int len = (int)strlen(in);
        if (80 < len) flag = false;
        // üÁ½lª80Èã¾Á½çAflagð0É·é
        else {
            max_len = len;
            int j = 0;
            for (int i = (len - 1); -1 < i; i--){
                num1[j] = in[i] - '0';
                j++;
            }
        }
        
        scanf("%s", in);
        // êUlðæèÞ
        len = (int)strlen(in);
        if (80 < len) flag = false;
        // üÁ½lª80Èã¾Á½çAflagð0É·é
        else {
            if (max_len < len) max_len = len;
            int j = 0;
            for (int i = (len - 1); -1 < i; i--){
                num2[j] = in[i] - '0';
                j++;
            }
        }
        
        // üÁ½lª80Èã¶áÈ¢Æ«ÉZ
        if (flag){
            for (int i = 0; i < 80; i++){
                num1[i] += num2[i];
                if (10 <= num1[i]){
                    num1[i + 1] += 1;
                    num1[i] %= 10;
                }
            }
            
            if (num1[80] != 0){
                flag = 0;
                // 81Úª0¶áÈ¢ = overflow
            }
            
            // overflowµÄÈ¢Æ«ÉoÍ
            if (flag){
                // ÅãÌ«µã°ÅJèãªèµÄé©Ç¤©
                if (num1[max_len] == 0){
                    for (int i = max_len - 1; -1 < i; i--){
                        printf("%d", num1[i]);
                    }
                    printf("\n");
                }
                else {
                    for (int i = max_len; -1 < i; i--){
                        printf("%d", num1[i]);
                    }
                    printf("\n");
                }
            }
        }
        
        // ¡ÜÅÉflagª0ÉÈÁÄ½çoverflowI
        if (!flag){
            printf("overflow\n");
        }
        
        n--;
    }
    
}

C++: #include<iostream>
#include<string>
#include<iomanip>
#include<cmath>
using namespace std;

int main(){

	int n;

	string a, b;
	char N1[80], N2[80];
	char sum[80], temp[81];
	string ans;

	cin >> n;

	for( int i=0;i<n;i++ ){

		cin >> a;
		
		cin >> b;

		if( a.length() >= 80 ){
			cout << "overflow" << endl;
			continue;
		}

		if( b.length() >= 80 ){
			cout << "overflow" << endl;
			continue;
		}

		for( int i=0;i<80;i++ ){
			N1[i] = -1; N2[i] = -1; sum[i] = 0;
		}

		for( int i=1;i<=a.length();i++ )
			N1[i] = a[a.length()-i] - '0';
		for( int i=1;i<=b.length();i++)
			N2[i] = b[b.length()-i] - '0';

		int over = 0;
		
		for( int i=0;i<80;i++ ){


			if( N1[i] == -1 && N2[i] == -1 ){
				if( over >= 10 ){
					sum[i] = over / 10;
					over = 0;
				}else
					sum[i] = -1;
				continue;
			}

			if( N1[i] != -1 )
				sum[i] += N1[i];
			if( N2[i] != -1 )
				sum[i] += N2[i];

			sum[i] += over / 10;

			over = floor((double)sum[i]/10)*10;

			sum[i] -= over;
			
		}

		if(over > 0)cout << "overflow" << endl;
		else{
			int j=0;
			for( int i=1;i<=80;i++ ){
				if( sum[80-i] >= 0 ){
					temp[j] = sum[80-i] + '0';
					j++;
					continue;
				}
				
			}
			temp[j] = '\0';

			ans = temp;

			cout << ans << endl;
		}

	}
	return 0;
}