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 <cstdlib>
#define x 81
using namespace std;

int main(){
	int n;
	cin >> n;
	cin.ignore();
	for(int i=0;i<n;i++){
		string a,b;
		getline(cin,a);
		getline(cin,b);
		bool c = false;
		if(a.size()>=x) c = true;
		if(b.size()>=x) c = true;
		int p[x];
		int k = 0;
		for(int j=0;j<x;j++){
			p[j] = k;
			int s = a.size()-j-1;
			int t = b.size()-j-1;
			string f;
			string g;
			if(s>=0) f = a.substr(s,1);
			if(t>=0) g = b.substr(t,1);
			if(s>=0) p[j] += atoi(f.c_str());
			if(t>=0) p[j] += atoi(g.c_str());
			if(p[j]>=10){ p[j] -= 10; k = 1; }
			else k = 0;
		}
		if(k==1) c = true;
		if(c) cout << "overflow" << endl;
		else{
			int d = x-1;
			while(p[d]==0){ d--; }
			for(int j=d;j>=0;j--) cout << p[j];
			cout << endl;
		}
	}
}

C++: 
#include <iostream>
#include <cstdlib>
#include <cstdio>
#include <cmath>
#include <stack>
#include <cctype>
#include <complex>
#include <vector>
#include <algorithm>
#include <cstring>		//strlen

using namespace std;



bool solve(){

//	なぜかわからないがstirngだと数字が大きいとSegmentation fault
/*
	string str1,str2;
	cin>> str1>> str2;
	int n1 = str1.size();
	int n2 = str2.size();
*/	
	char str1[10000],str2[10000];
	scanf("%s %s",str1,str2);
	int n1 = strlen(str1);
	int n2 = strlen(str2);
	
	int n  = max(n1,n2);

/*	string str="";
*/
	char str[10000];
	int c = 0;
	int i=0;
	while(i<n){
		int a = c;
		if(i<n1){
			a += (str1[n1-i-1]-'0');
		}
		if(i<n2){
			a += (str2[n2-i-1]-'0');
		}
		c = a/10; 
		a = a%10;
		
		str[i] = a + '0';
		i++;
	}
	if(c){
		str[i] = c + '0';
		i++;
	}
	
	if(i>=81){
		cout<< "overflow"<<endl;
		return true;
	}
	
	for(int j=i-1;j>=0;j--){
		cout<< str[j];
	}
	cout<<endl;
	
	return true;
}

int main(){
	cout.setf(ios::fixed);
	cout.precision(5);
	int n;
	cin>> n;
	for(int i=0;i<n;i++){
		solve();
	}
	return 0;
}

C++: #include <cstdio>
#include <cmath>
#include <cstring>
#include <iostream>

using namespace std;

const int MAX_INPUT = 1000000;
const int ARRAY_SIZE = 9;
const int MAX_LEN = 8;
char inputA[MAX_INPUT];
char inputB[MAX_INPUT];

void toBigInt(char *str, long *r) {
    int len = strlen(str);
    int startIndex = ARRAY_SIZE - (len / MAX_LEN) - 1;

    {
        int offset = 0;
        for (int i = 0; i < ARRAY_SIZE; ++i) {
            char temp[MAX_LEN + 1] = { 0 };
            char* start = str + ((i == 0) ? 0 : offset);
            int readLen = ((len - offset) < MAX_LEN) ? len - offset : MAX_LEN;

            strncpy(temp, start, readLen);
            r[ARRAY_SIZE - i - 1] = (long)atoi(temp);
            offset += readLen;
        }
    }
}

int main() {
    while (scanf("%s\n%s", &inputA, &inputB) != EOF) {
        long a[ARRAY_SIZE] = { 0 };
        long b[ARRAY_SIZE] = { 0 };
        long r[10] = { 0 };
        int lenA = 0;
        int lenB = 0;
        const int MAX_INPUT_LEN = 80;
        lenA = strlen(&inputA[0]);
        lenB = strlen(&inputB[0]);
        if ((MAX_INPUT_LEN < lenA) || (MAX_INPUT_LEN < lenB)) {
            printf("overflow\n");
            continue;
        }

        toBigInt(inputA, a);
        toBigInt(inputB, b);
        for (int i = 0; i < ARRAY_SIZE; ++i) {
            int curRIndex = 10 - i - 1;
            r[curRIndex] += a[ARRAY_SIZE - i - 1] + b[ARRAY_SIZE - i - 1];
            const int MAX = 99999999;
            if (MAX < r[curRIndex]) {
                r[curRIndex - 1] = r[curRIndex] / (MAX + 1);
                r[curRIndex] %= (MAX + 1);
            }
        }

        {
            bool started = false;
            bool noOutput = true;
            for (int i = 0; i < 10; ++i) {
                if (!started && (0 < r[i])) {
                    started = true;
                    printf("%d", r[i]);
                    noOutput = false;
                    continue;
                }

                if (started) {
                    long temp = (r[i] == 0) ? 1 : r[i];
                    for (int j = 0; temp * pow(10, j) < 10000000; ++j) {
                        printf("0");
                    }

                    printf("%d", r[i]);
                }
            }

            if (noOutput) {
                printf("0");
            }
        }
        printf("\n");
    }

    return 0;
}