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 <sstream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <ctime>
#include <cstring>
#include <climits>
#include <algorithm>
#include <map>
#include <set>
#include <stack>
#include <vector>
#include <queue>
#include <string>
#include <complex>
using namespace std;

#define rep(i,n) for(int i=0; i<n; i++)
#define repa(i,s,e) for(int i=s; i<=e; i++)
#define repd(i,s,e) for(int i=s; i>=e; i--)

typedef vector<int> vi;

int n;
string s1, s2;
char n1[80], n2[80], sum[80];

int main() {
	scanf("%d", &n);
	rep(i,n) {
		memset(n1, 0, sizeof(n1));
		memset(n2, 0, sizeof(n2));
		memset(sum, 0, sizeof(sum));

		cin>>s1>>s2;
		int l1 = s1.size();
		int l2 = s2.size();
		if(max(l1,l2) > 80) {
			printf("overflow\n");
			return 0;
		}

		rep(k,l1) n1[k] = s1[l1-k-1] - '0';
		rep(k,l2) n2[k] = s2[l2-k-1] - '0';

		char a = 0;
		rep(i,80) {
			char c = n1[i] + n2[i] + a;
			a = c / 10;
			sum[i] = c % 10;
		}

		if(a != 0) {
			printf("overflow\n");
			return 0;
		}

		bool start = false;
		repd(i,79,0) {
			if(sum[i] != 0) start = true;
			if(!start && sum[i] == 0) continue;
			printf("%d", sum[i]);
		}
		if(!start) printf("0");
		printf("\n");
	}
}

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

using namespace std;

int main() {
    int n;
    cin >> n;
    while (n--) {
        int len, keta = 0;
        string a, b, c = "";
        cin >> a >> b;
        reverse(a.begin(), a.end());
        reverse(b.begin(), b.end());
        len = max(a.size(), b.size());
        for (int i = 0, size = len - a.size(); i < size; i++) {
            a += '0';
        }
        for (int i = 0, size = len - b.size(); i < size; i++) {
            b += '0';
        }
        for (int i = 0; i < len; i++) {
            c += ((a[i] - '0') + (b[i] - '0') + keta) % 10 + '0';
            keta = ((a[i] - '0') + (b[i] - '0') + keta) / 10;
        }
        if (keta == 1) {
            c += '1';
        }
        reverse(c.begin(), c.end());
        if (c.size() > 80) {
            cout << "overflow\n";
        }
        else {
            cout << c << endl;
        }
    }
    
    return 0;
}

C++: #include <stddef.h>
#include <algorithm>
#include <iostream>
#include <iterator>
#include <string>
 
using namespace std;
int main() {
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    string s1, s2, s3 = "";
    cin >> s1 >> s2;
    size_t m = max(s1.size(), s2.size());
    while (s1.size() < m)
      s1.insert(s1.begin(), '0');
    while (s2.size() < m)
      s2.insert(s2.begin(), '0');
    char c = 0;
    auto it1 = s1.rbegin();
    auto it2 = s2.rbegin();
    for (; it1 != s1.rend(); it1++, it2++) {
      char add = *it1 - '0' + *it2 - '0' + c;
      char ch = add % 10 + '0';
      c = add / 10;
      s3.insert(s3.begin(), ch);
    }
    if (c) s3.insert(s3.begin(), c + '0');
    cout << ((s3.size() > 80) ? "overflow" :s3) << endl;
  }
}

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;
       if (str_a.size() > 80 || str_b.size() > 80 || sum.size() > 80){
           cout << "overflow" << endl;}else{
   cout << sum << endl;}
   sum = "";
   }
}

C++: #include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main(void)
{
	char num1[1002],num2[1002];
	int a1[1002],a2[1002],ans[1002];
	int i,j,n,k,l1,l2,kuriagari=0;
	scanf("%d",&n);
	for(i=0;i<n;i++)
	{
		kuriagari=0;
		for(j=0;j<1002;j++)
		{
			num1[j]='\0';
			num2[j]='\0';
			a1[j] = 0;
			a2[j] = 0;
			ans[j] = 0;
		}
		scanf("%s",num1);
		getchar();
		scanf("%s",num2);
		getchar();
		l1 = strlen(num1);
		l2 = strlen(num2);
		//printf("%s %s\n",num1,num2);
		for(j=0;j<l1;j++)
		{
			a1[j] = num1[l1-1-j] - '0';
		}
		for(j=0;j<l2;j++)
		{
			a2[j] = num2[l2-1-j] - '0';
		}
		for(j=0;j<1002;j++)
		{
			ans[j] = (a1[j] + a2[j] + kuriagari) % 10;
			if((int)((a1[j] + a2[j] + kuriagari)/10) == 1)
			{
				kuriagari = 1;
			}
			else
			{
				kuriagari = 0;
			}
		}
		bool k=false;
		for(j=1001;j>=80;j--)
		{
			if(ans[j]!=0)
			{
				k=true;
				break;
			}
		}
		if(k)
		{
			printf("overflow\n");
			continue;
		}
		else
		{
			for(j=1001;j>=0;j--)
			{
				if(ans[j] != 0)
				{
					k = j;
					break;
				}
			}
			for(j=k;j>=0;j--)
			{
				printf("%d",ans[j]);
			}
			printf("\n");
		}
	}
	return 0;
}