Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	sc.Scan()
	dataset, _ := strconv.Atoi(sc.Text())
	var firstdata *big.Int

	for i := 0; i < dataset*2; i++ {
		n := new(big.Int)
		sc.Scan()
		data, _ := n.SetString(sc.Text(), 10)

		if i%2 == 0 {
			firstdata = data
			continue
		}

		result := new(big.Int)
		if result := result.Add(data, firstdata); len(result.String()) > 80 {
			fmt.Println("overflow")
			continue
		} else {
			fmt.Println(result)
		}
	}
}

C++: #include <iostream>
#include <string>
#include <sstream>
#include <algorithm>
#include <vector>
#include <utility>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <ctime>
#include <cstdlib>
#include <map>
#include <set>
#include <stack>
#include <queue>
 
using namespace std;
 
#define pb(n)	push_back(n)
#define mp(n,m) make_pair(n,m)
#define fi 	first
#define se 	second
#define all(r) (r).begin(),(r).end()
 
#define REP(i,s,e)	for(int i=(s); i<(e); i++)
#define rep(i,n)	REP(i,0,n)
#define REPE(i,s,e)	for(int i=s; i>e; i--)
#define repe(i,n)	REPE(i,n,-1)


typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
 
typedef vector<vi> vvi;
 
const int IMAX=((1<<30)-1)*2+1;
const int INF=100000000;
const double EPS=1e-10;
//int mod=1000000007
 

int main(){
	int n;
	cin>>n;
	rep(l,n){
		string s,t;
		cin>>s>>t;
		vi ans(80,0);
		if(s.size()>80||t.size()>80) break;
		for(int i=0; i<s.size(); i++){
			ans[80-s.size()+i]+=s[i]-'0';
		}
		for(int i=0; i<t.size(); i++){
			ans[80-t.size()+i]+=t[i]-'0';
		}
		for(int i=79; i>0; i--){
			if(ans[i]>=10) ans[i]-=10, ans[i-1]+=1;
		}
		if(ans[0]>=10) cout<<"overflow"<<endl;
		else{
			bool f=false;
			rep(i,80){
				if(f||(!f && ans[i])||i==79){
					cout<<ans[i];
					f=true;
				}
			}
			cout<<endl;
		}
	}
}

C++: #include<bits/stdc++.h>
#define rep(i,n) for(int i=0;i<n;++i)
#define FOR(i,a,b) for(int i=a;i<=b;++i)
#define LL long long
using namespace std;
static const LL INF = 1LL<<61LL;
typedef pair<int,int> PII;

string a,b;
string ans;
int M;
char t[]={'0','1','2','3','4','5','6','7','8','9'};

void add(int n,int m,int c,string s){
    int k;
    if(n-1<0&&m-1<0){
        if(c!=0)s+=t[c];
        for(int i=s.size()-1;i>=0;--i){
            ans+=s[i];
        }
        return ;
    }
    if(n-1>=0&&m-1>=0)k=a[n-1]-'0'+b[m-1]-'0'+c;
    else if(n-1>=0)k=a[n-1]-'0'+c;
    else if(m-1>=0)k=b[m-1]-'0'+c;
    if(k>=10){
        k-=10;
        c=1;
    }
    add(n-1,m-1,c,s+t[k]);
}

/*bool MOD1(string num){
    bool res=false;
    LL k=num.size();
    LL mod;
    int i=-1;
    LL n=0;
    while(k){
        n=(n*10)+num[i+1]-'0';
        mod=n%M;
        n=mod;
        k--;
        i++;
    }
    if(mod==0)return true;
    return false;
}
bool MOD2(string num){
    bool res=false;
    LL k=num.size();
    LL mod;
    int i=-1;
    LL n=0;
    while(k){
        n=(n*10)+num[i+1]-'0';
        mod=n%5;
        n=mod;
        k--;
        i++;
    }
    if(mod==0)return true;
    return false;
}
bool MOD3(string num){
    bool res=false;
    LL k=num.size();
    LL mod;
    int i=-1;
    LL n=0;
    while(k){
        n=(n*10)+num[i+1]-'0';
        mod=n%7;
        n=mod;
        k--;
        i++;
    }
    if(mod==0)return true;
    return false;
}
bool MOD4(string num){
    bool res=false;
    LL k=num.size();
    LL mod;
    int i=-1;
    LL n=0;
    while(k){
        n=(n*10)+num[i+1]-'0';
        mod=n%11;
        n=mod;
        k--;
        i++;
    }
    if(mod==0)return true;
    return false;
}*/
int main(){
    int n;
    cin>>n;
    rep(i,n){
    cin>>a>>b;
    //cin>>M;
        add(a.size(),b.size(),0,"");
        //if(!MOD1(ans)&&!MOD2(ans)&&!MOD3(ans)&&!MOD4(ans))cout<<ans<<endl;
        //else cout<<"king is angry"<<endl;
        if(ans.size()<=80)cout<<ans<<endl;
        else cout<<"overflow"<<endl;
        ans.clear();
    }
}

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

int main(){
 int n;
 scanf("%d\n",&n);
 for(int x = 0;x < n;x++){
  char A[101],B[101];
  scanf("%s\n%s\n",A,B);
  
  int l_a,l_b;
  if((l_a = strlen(A)) > 80 || (l_b = strlen(B)) > 80){
   printf("overflow\n");
  }
  int val[81];
  int i;
  for(i = 0;i < l_a || i <l_b;i++){
   if(l_a - i > 0) val[i] += A[l_a - i - 1] - '0';
   if(l_b - i > 0) val[i] += B[l_b - i - 1] - '0';
  }
  bool flg = false;
  for(int j = i;j >= 0;j--){
   if(val[j] >= 10){
    if(j - 1 < 0) flg = true;
    else          val[j] -= 10,val[j - 1]++;
   }
  }
  if(flg){
   printf("overflow\n");
  }else{
   for(;i >= 0;i--){
    printf("%d",val[i]);
   }
   printf("\n");
  }
 }
 return 0;
}

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

int main(){
  string s,t, u;
  int n;
  char ch;

  cin >> n;
  for( int loop=0;loop<n;loop++ ){
    cin >> s >> t;

    //    cout << "\ns=" << s << endl << "t=" <<  t << endl;

    if( s.length()>80 || t.length()>80 ){
      cout << "overflow" << endl;
      continue;
    }

    while( s.length()>t.length() )
      t = "0" + t;
    while( t.length()>s.length() )
      s = "0" + s;
    /*
    cout << "s=" << s << " len=" << s.length() << endl;
    cout << "t=" << t << " len=" << t.length() << endl;
    */
    u="";
    int c=0;
    for( int i=s.length();i>=0;--i ){
      int a = s[i]-'0';
      int b = t[i]-'0';
      int sum = a + b + c;
      if( sum>=10 ){
	sum-=10; c=1;
      }else
	c=0;
      ch = sum + '0';
      u = ch + u;
    }
    if( c==1 )
      u = "1" + u;

    if( u.length()>80 )
      cout << "overflow" << endl;
    else
      cout << u << endl;
  }
  return 0;
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <string.h>


//¶ñðtÉ·éÖ
void reverseString(char *str){
	char *hoge;
	int length;

	hoge = (char *)malloc( sizeof(char) * strlen(str) );
	
	length = strlen(str);
	for(int i=0 ; i<length ; i++){
		hoge[length-i-1] = str[i];
	}
	for(int i=0 ; i<length ; i++){
		str[i] = hoge[i];
	}
	free(hoge);
}

int main(){
	char a[1000], b[1000], c[1000];
	int n, num1, num2, index;
	bool flag, printFlag;

	//f[^ZbgÌüÍ
	scanf("%d", &n);
	
	for(int i=0 ; i<n ; i++){
		//ÏÌú»
		for(int j=0 ; j<1000 ; j++){
			a[j] = 0;
			b[j] = 0;
			c[j] = 0;
		}
		flag = false;

		//QÂÌlðüÍ(¶ñÆµÄzñÉi[)
		scanf("%s", a);
		scanf("%s", b);

		//QÂÌlÌªáÁÄàvZÅ«éæ¤É¶ñðtÉ·é
		reverseString(a);
		reverseString(b);
		//QÂÌlª80ð´¦Ä¢½çtOð§Äé
		if( strlen(a)>80 || strlen(b)>80){
			flag = true;
		}
		
		//aðvZ·é
		for(int j=0 ; j<100 ; j++){
			if(flag) break;

			//48ø¢Ä¶ñ©çlÉÏ··é
			if(a[j]>=48)a[j] -= 48;
			if(b[j]>=48)b[j] -= 48;

			if( (a[j]+b[j]+c[j]) >= 10 ){//Jèãªè
				c[j+1]++;
			}
			c[j] += (a[j]+b[j])%10;
			c[j] %= 10;
		}
		//ÅãÊª½Ú©²×é
		for(int j=79 ; j>=0 ; j--){
			if(c[j] != 0){
				index = j;
				break;
			}
		}
		for(int j=0 ; j<index ; j++){
			if(c[j]==0)c[j] += 48;
		}
		reverseString(c);
		if(strlen(c)>80){
			flag = true;
		}
		for(int j=0 ; j<=index ; j++){
			if( flag==false ){
				if(c[j]==48) c[j] = 0;
				printf("%c", c[j]+48);
			}
		}
		if(flag){
			printf("overflow");
		}
		printf("\n");
	}
	return 0;
}