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 <cmath>
#include <cstdlib>

using namespace std;

int main(){
	int N; cin >> N;
	for(int i = 0; i < N; i++){
		long double a, b;
		cin >> a >> b;
		if(log10(a + b) > 80) cout << "overflow" << endl, 
		exit(2);
		cout << a + b << endl;
	}
	return 0;
}

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

using namespace std;
int main(){
    string inp[2];
    int output[81],n;
    cin >> n;
    for(int j=0;j<n;j++){
        fill(output,output+81,0);
        cin >> inp[0] >> inp[1];
        if(inp[0].length()>80||inp[1].length()>80) cout << "overflow" << endl;
        else{
            for(int i=0;i<81;i++){
                if((int)inp[0].length()-1-i >= 0){
                    output[i]+=inp[0][inp[0].length()-1-i]-'0';
                }
                if((int)inp[1].length()-1-i >= 0){
                    output[i]+=inp[1][inp[1].length()-1-i]-'0';
                }
                if(output[i]>9){
                    output[i+1]+=output[i]/10;
                    output[i]%=10;
                }
            }
            if(output[80]>0) cout << "overflow" << endl;
            else{
                bool flag=false;
                for(int i=79;i>=0;i--){
                    if(!flag&&output[i]!=0) flag=true;
                    if(flag){
                        cout << output[i];
                    }
                }
                cout << endl;
            }
        }
    }
    return 0;
}
//90000000000000000000000000000000000000000000000000000000000000000000000000000000

C++: #include <iostream>
#include <cstdlib>
using namespace std;

int main(){


  string strx,stry;
  int N;

  cin >> N;
  while(N){
    cin >> strx ;
    cin >> stry;
    int x[20] = {0},y[20] = {0};
    int i=0,lenx = strx.size(), leny = stry.size();
    string tmp;
    
    for( i = 0; lenx > 5 ; i++){
      tmp = strx.substr(lenx - 5);
      x[i] = atoi(tmp.c_str());
      strx =  strx.substr(0,lenx - 5);
      lenx -= 5;
    }
    x[i] = atoi(strx.c_str());
    for( i = 0; leny > 5 ; i++){
      tmp = stry.substr(leny - 5);
      y[i] = atoi(tmp.c_str());
      stry =  stry.substr(0,leny - 5);
      leny -= 5;
    }
    y[i] = atoi(stry.c_str());
    
    
    int ans[20] = {0};
    for(i = 0 ; i < 20 ; i++){
      ans[i] += x[i] + y[i];
      if(ans[i] > 99999){
	ans[i+1] += 1;
	ans[i] -= 100000;
      }
    }
    if(ans[16] != 0)
      cout << "overflow"<< endl;
    else{
      for(i = 15; i >= 0 ; i--){
	if(ans[i] != 0)
	  break;
      }
      cout << ans[i];
      for(i = i-1; i >= 0 ; i--){
	if(ans[i] < 10) cout << "0";
	if(ans[i] < 100) cout << "0";
	if(ans[i] < 1000) cout << "0";
	if(ans[i] < 10000) cout << "0";
	cout << ans[i];
      }
      cout << endl;
    }
    N--;
  }
  return 0;
}

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

using namespace std;

int main (int argc, char * const argv[]) {
    // insert code here...
	int n;
    string a, b;
		
	cin>> n;	
	for(int i=0;i<n;i++){
		int x[81]={0}, y[81]={0}, z[81]={0};

		cin >> a;
		cin >> b;
		
		reverse(a.begin(), a.end());
		reverse(b.begin(), b.end());
		
		for(int j=0;j<a.size();j++){
			x[j]=a[j]-'0';
		}
		for(int j=0;j<b.size();j++){
			y[j]=b[j]-'0';
		}
		
		for(int j=0;j<80;j++){
			z[j]+=x[j]+y[j];
			if(z[j]>=10){
				z[j+1]++;
				z[j]-=10;
			}
		}
		
		if(z[80]>0){
			cout << "overflow" <<endl;
		}else{
			bool flag = 0;
			for(int j=79;j>=0;j--){
				if(z[j]==0&& flag==0){
					continue;
				}else{
					flag=1;
				}
				cout<<z[j];
			}
			if(flag==0){
				cout<<0;
			}
			cout<<endl;
			
		}
		
	}
	
	return 0;
}