Go: package main

import (
	"bufio"
	"fmt"
	"math/big"
	"os"
	"strconv"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

func main() {
	scanner.Split(bufio.ScanWords)

	const maxCapacity = 512 * 1024
	buf := make([]byte, maxCapacity)
	scanner.Buffer(buf, maxCapacity)

	n := nextInt()
	maxint, _ := new(big.Int).SetString("100000000000000000000000000000000000000000000000000000000000000000000000000000000", 10)

	for i := 0; i < n; i++ {
		aStr := nextString()
		bStr := nextString()

		a, _ := new(big.Int).SetString(aStr, 10)
		b, _ := new(big.Int).SetString(bStr, 10)
		c := big.NewInt(0).Add(a, b)

		if c.Cmp(maxint) < 0 {
			fmt.Println(c)
		} else {
			fmt.Println("overflow")
		}
	}
}

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

using namespace std;

int main(void)
{
  int n=0;
  string as;
  string bs;
  int a[100]={};
  int b[100]={};
  int c[100]={};
  int i=0;
  int alen=0;
  int blen=0;
  int clen=0;
  int k=0;
  int j=0;

  cin>>n;

  for(j=0;j<n;j++)
    {
      for(i=0;i<100;i++)
        a[i]=b[i]=c[i]=0;
      alen=0;
      blen=0;
      clen=0;
      k=0;

      cin>>as;
      cin>>bs;

      alen=as.size();
      blen=bs.size();

      for(i=0;i<alen;i++)
        {
          a[i]=as[alen-i-1];
          //printf("a[%d]=%d\n", i, a[i]);                                      
          switch(a[i])
            {
            case '0' : a[i]=0; break;
            case '1' : a[i]=1; break;
            case '2' : a[i]=2; break;
            case '3' : a[i]=3; break;
            case '4' : a[i]=4; break;
            case '5' : a[i]=5; break;
            case '6' : a[i]=6; break;
            case '7' : a[i]=7; break;
            case '8' : a[i]=8; break;
            case '9' : a[i]=9; break;
            }
        }

      for(i=0;i<blen;i++)
        {
          b[i]=bs[blen-i-1];
          //printf("b[%d]=%d\n", i, b[i]);                                      
          switch(b[i])
            {
            case '0' : b[i]=0; break;
            case '1' : b[i]=1; break;
            case '2' : b[i]=2; break;
            case '3' : b[i]=3; break;
            case '4' : b[i]=4; break;
            case '5' : b[i]=5; break;
            case '6' : b[i]=6; break;
            case '7' : b[i]=7; break;
            case '8' : b[i]=8; break;
            case '9' : b[i]=9; break;
            }
        }

      clen=(alen > blen)? alen : blen;

      for(i=0;i<clen;i++)
        {
          c[i]+=a[i]+b[i];
          c[i+1]=c[i]/10;
          if(c[clen]!=0)
            k=1;
          c[i]=c[i]%10;
        }

      if(clen>80 || c[80]!=0)
        cout<<"overflow"<<endl;

      else
        {
          for(i=clen+k-1;i>=0;i--)
            cout<<c[i];
          puts("");
        }
    }

}

C++: #include<iostream>
#include<string>
using namespace std;
int main(){
  int n;
  cin >>n;
  while(n--){
    string a,b,ans;
    cin >>a>>b;
    int as = a.size(), bs = b.size(), ms = (as>bs)? as:bs,t=0;
    for(int i=0; i<ms; i++){
      if(i<as && i<bs){t = a[as-i-1]+b[bs-i-1]+t/10-2*'0';}
      else if(i<as){t = a[as-i-1]+t/10-'0';}
      else if(i<bs){t = b[bs-i-1]+t/10-'0';}
      ans+=(char)(t%10)+'0';
    }
    if(t/10){ans+='1';}
    if(ans.size()>80){cout <<"over flow"<<endl;}
    else{for(int i=0; i<ans.size(); i++){cout <<ans[ans.size()-i-1];}cout <<endl;}
  }
  return 0;
}

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) + 1 > 80){
			cout << "overflow" << endl; 
		}
		cout << a + b << endl;
	}
	return 0;
}

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

using namespace std;
typedef long long ll;

#define PI 3.141592653

int main(){
    string A,B;
    int n,Ak,Bk,ans[81];
    
    scanf("%d",&n);
    
    for(int i=0;i<n;i++){
        
        for(int j=0;j<80;j++){
            ans[j]=0;
        }
        
        cin >> A >> B;
        
        if(A==B && A=="0"){
            printf("0\n");
            continue;
        }
        
        Ak=(int)A.size();
        Bk=(int)B.size();
        
        if(Ak>80 || Bk>80){
            puts("overflow");
            continue;
        }
        
        for(int j=0;j<max(Ak,Bk);j++){
            if(j<min(Ak,Bk))
                ans[79-j]+=(A[Ak-1-j]-'0')+(B[Bk-1-j]-'0');
            else if(j<Ak)
                ans[79-j]+=A[Ak-1-j]-'0';
            else if(j<Bk)
                ans[79-j]+=B[Bk-1-j]-'0';
            
            if(ans[79-j]>=10){
                if(j==79){
                    break;
                }
                ans[79-j-1]++;
                ans[79-j]-=10;
            }
        }
        
        if(ans[0]>=10){
            puts("overflow");
        }else{
            bool f = false;
            for(int j=0;j<80;j++){
                if(ans[j]>0 || f){
                    printf("%1d",ans[j]);
                    f = true;
                }
            }
            printf("\n");
        }
    }
    
    return 0;
}

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


int main(){
  int n,i,j;
  string a,b;
  char sum[110]={};
  fill(sum,sum+110,'0');
  int up = 0;
  cin >> n;
  for(i=0;i<n;i++){
    cin >> a >> b;
    int sa=a.size(),sb=b.size();
    int flag=0;
    if(sa>=80||sb>=80) flag =1;
    for(j=0;j<max(sa,sb);j++){
      if(sa>j&&sb>j){
	sum[j] = (a[sa-j-1]-'0') + (b[sb-j-1]-'0') + up;
      }else if(sa>j){
	sum[j] = (a[sa-j-1]-'0') + up;
      }else if(sb>j){
	sum[j] = (b[sb-j-1]-'0') + up;
      }
      up=0;
      if(sum[j]>9){
	up=1;
	sum[j]-=10;
      }
      sum[j]+='0';
    }
    sum[max(sa,sb)]=up;
    int k=110-1;
    while(sum[k]-'0'==0&&k>0) k--;
    if(k>=80||flag==1) cout << "overflow";
    else
    for(j=k;j>=0;j--)
      cout << sum[j];
    cout << endl;
  }
}

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

int main(){
	string s1,s2;
	int num;
	cin>>num;
	for(int j=0;j<num;j++){
		cin>>s1;
		cin>>s2;
		int size1=s1.size();
		int size2=s2.size();
		if(size1>80 || size2>80){
			cout<<"overflow"<<endl;
			continue;
		}
		if(size1>size2){
			for(int a=0;a<(size1-size2);a++){
				s2='0'+s2;
			}
		}else if(size1<size2){
			for(int a=0;a<(size2-size1);a++){
				s1='0'+s1;
			}
		}
		string ans="";
		int plus=0;
		for(int i=max(size1,size2)-1;i>=-1;i--){
			if(i==-1){
				if(plus!=0){
					ans=(char)(plus+'0')+ans;
					continue;
				}else{
					continue;
				}
			}
			int alt=(s1[i]-'0')+(s2[i]-'0')+plus;
			if(alt>=10){
				ans=(char)(alt-10+'0')+ans;
				plus=alt/10;
			}else ans=(char)(alt+'0')+ans;
		}
		if(ans.size()>80){
			cout<<"overflow"<<endl;
		}else{
			cout<<ans<<endl;
		}
	}
	return 0;
}