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 <stdio.h>
#include <string.h>
 
int main(void) {
    char a[200],b[200],result[200],temp[200];
    int ketaagari;
    int i,j;
    int max;
    char in;
    int overflow;
    int count,kazu;
    scanf("%d",&kazu);
    for(count=0;count<kazu;count++) {
        for(i=0;i<200;i++) {
            a[i]=0;b[i]=0;
        }
        i=0;
        overflow=0;
    	
    	scanf(" %s ",temp);
    	i=strlen(temp);
    	if(i>80)overflow=1;
        else for(j=0;j<i;j++)a[j]=temp[i-j-1]-'0';
    	
    	scanf(" %s ",temp);
    	i=strlen(temp);
    	if(i>80)overflow=1;
    	else for(j=0;j<i;j++)b[j]=temp[i-j-1]-'0';
 
        for(i=0,ketaagari=0,max=0;i<80;i++) {
            result[i]=(a[i]+b[i]+ketaagari)%10;
            ketaagari=(a[i]+b[i]+ketaagari)/10;
            if(result[i]!=0)max=i;
        }
        if(ketaagari)overflow=1;

    	if(overflow==0) {
            for(i=max;i>=0;i--)printf("%d",result[i]);
            printf("\n");
        } else printf("overflow\n");
    }
    return 0;
}

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

using namespace std;

void reverse(string *s){
	for(int i=0;i<s->length()/2;i++){
		char tmp=s->at(i);
		s->at(i)=s->at(s->length()-i-1);
		s->at(s->length()-i-1)=tmp;
	}
}

int main(void){
	int n;
	cin>>n;
	for(int i=0;i<n;i++){
		string s1,s2;
		cin>>s1;
		cin>>s2;
		if(s1.length()>80||s2.length()>80){
			cout<<"overflow"<<endl;
		}else{
			string s3;
			string tmp;
			if(s1.length()<s2.length()){
				tmp=s1;
				s1=s2;
				s2=tmp;
			}
			int c=0;
			for(int j=0;j<s1.length();j++){
				int tmp;
				if(j<s2.length()){
					tmp=s1[s1.length()-j-1]-'0'+s2[s2.length()-j-1]-'0'+c;
				}else{
					tmp=s1[s1.length()-j-1]-'0'+c;
				}
				c=tmp/10;
				s3.append(1,tmp%10+'0');
			}
			if(c!=0){
				s3.append(1,'1');
			}
			if(s3.length()>80){
				cout<<"overflow"<<endl;
			}else{
				reverse(&s3);
				cout<<s3<<endl;
			}
		}
	}
}

C++:  #include<iostream>
 #include<stdio.h>
 #include<string>
 #include<math.h>
 #include<iomanip>
 #include<algorithm>
 #include<string.h>
 #include<cctype>
 #include<map>
 #include<set>
 #include<vector>
 #include<sstream>
 #include<stack>
 
 using namespace std;
 
 int main()
 {
   int N;
   cin>>N;
   while(N--)
   {
     string a,b,sum;
     cin>>a>>b;
     bool flag=false;
     if(a.length()>80||b.length()>80) flag=true;
     else
     {
       int up=0;
       int i;
       for(int j=0;j<a.length()/2;j++) swap(a[j],a[a.length()-j-1]);
       for(int j=0;j<b.length()/2;j++) swap(b[j],b[b.length()-j-1]);
       for(i=0;i<a.length()&&i<b.length();i++)
       {
         sum+=(((a[i]-'0')+(b[i]-'0')+up)%10)+'0'; 
         if(a[i]-'0'+b[i]-'0'+up>=10) up=1;   
         else up=0;  
       }          
       while(i<a.length()) 
       {
         sum+=((a[i]-'0'+up)%10)+'0';
         if(a[i]-'0'+up>=10) up=1; 
         else up=0;         
         i++;          
       }
       while(i<b.length()) 
       {
         sum+=((b[i]-'0'+up)%10)+'0';
         if(b[i]-'0'+up>=10) up=1; 
         else up=0;
         i++;                   
       }
       if(up==1) sum+='1';
       if(sum.length()>80) flag=true;
     }
     if(flag) cout<<"overflow"<<endl;
     else
     {
       for(int i=sum.length()-1;i>=0;i--) cout<<sum[i];
       cout<<endl;    
     }
   }
   //while(1);      
   return 0;    
 }

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

char iota(int n){
  switch (n) {
    case 0: return '0';
    case 1: return '1';
    case 2: return '2';
    case 3: return '3';
    case 4: return '4';
    case 5: return '5';
    case 6: return '6';
    case 7: return '7';
    case 8: return '8';
    case 9: return '9';
  }
  return '0';
}

int main(){

  std::string num1;   //("10000");
  std::string num2;   //("990000");
  char sum[100]={};
  int n,i,j,flag=0,temp;

  std::cin >> n;
  //n=1;
  for(i=0;i<100;i++){
    sum[i]='\0';
  }

  for(i=0;i<n;i++){
    std::cin >> num1 >> num2;
    if(num1.size()<num2.size()){
      num1.swap(num2);
    }
    //std::cout << num1 << std::endl;
    //std::cout << num2 << std::endl;
    for(j=0;j<num1.size();j++){
      if(j<num2.size()){
        temp = (num1[num1.size()-j-1]-'0')+(num2[num2.size()-j-1]-'0');
      }
      if(j>=num2.size()){
        temp = num1[num1.size()-j-1]-'0';
      }
      if(flag){
      ++temp;
      flag = 0;
      }
      if(temp>9){
        temp -= 10;
        flag = 1;
      }
      sum[j] = iota(temp);
    }
    if(flag){
      sum[j] = '1';
      flag = 0;
    }
    //printf("%s\n",sum);
    j=0;
    while(sum[j]!='\0'){
      ++j;
    }
    if(j>80){
      std::cout << "overflow" << std::endl;
    }else{
      while(j>0){
        std::cout << sum[j-1];
        --j;
      }
      std::cout << std::endl;
    }
  }
}