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 <algorithm>
using namespace std;
int main(){
  string num1,num2,tmp;
  int n;

  cin >> n;
  while(n--){

    cin >> num1 >> num2;

    if((int)num1.size() < (int)num2.size()){
      tmp = num2;
      num2 = num1;
      num1 = tmp;
    }

    if(num1.size() > 80){
      cout << "overflow" << endl;
      continue;
    }

    for(int i = 0 ; i < (int)num1.size() ; i++ )tmp[i] = num1[i];

    for(int i = (int)num1.size() - 1 ; i >= 0 ; i-- )
      {
	num1[i] = tmp[(int)num1.size() - 1 - i];
      }
    for(int i = 0 ; i < (int)num2.size() ; i++ )tmp[i] = num2[i];

    for(int i = (int)num2.size() - 1  ; i >= 0 ; i-- )
      {
	num2[i] = tmp[(int)num2.size() - 1 - i];
      }

    for(int i = (int)num2.size() ; i < (int)num1.size() ; i++ )num2+='0';

    char tmp2 = '0';
    string ans;

    for(int i = 0 ; i < num1.size() ; i++ ){
      if((tmp2 - '0') + (num1[i] - '0') + (num2[i] - '0') > 9){
	ans += (char)(((tmp2 - '0') + (num1[i] - '0') + (num2[i] - '0') - 10) + '0');
	tmp2 = '1';
      } else {
	ans += (char)(((tmp2 - '0') + (num1[i] - '0') + (num2[i] - '0')) + '0');
	tmp2 = '0'; 
      }
    }
  
    if((int)((tmp2 - '0') + (num1[num1.size() - 1] - '0') + (num2[num1.size() - 1] - '0')) > 9)ans+='1'; 
    if(ans.size() > 80){
      cout << "overflow" << endl;

    }else{

      for(int i = (int)ans.size() ; i >= 0 ; i-- ){
	cout << ans[i];
      }  
      cout << endl;
    }
  }

  return 0;
}

C++: #include<iostream>
#include<string>
#include<algorithm>
using namespace std;
int main(){
	int n;
	cin >> n;
	for(int i = 1; i <= n; i++){
		string a, b;
		cin >> a >> b;
		if(a.size() < b.size()) swap(a, b);
		reverse(a.begin(), a.end());
		reverse(b.begin(), b.end());
		string ans;
		bool carryover = false;
		for(int j = 0; j < b.size(); j++){
			int num;
			if(carryover) num = (a[j] - '0') + (b[j] - '0') + 1;
			else num = (a[j] - '0') + (b[j] - '0');
			if(num >= 10) carryover = true;
			else carryover = false;
			ans.push_back(num % 10 + '0');
		}
		for(int j = b.size(); j < a.size(); j++){
			int num;
			if(carryover) num = (a[j] - '0') + 1;
			else num = (a[j] - '0');
			if(num >= 10) carryover = true;
			else carryover = false;
			ans.push_back(num % 10 + '0');
		}
		if(carryover) ans.push_back('1');
		reverse(ans.begin(), ans.end());
		if(ans.size() > 80) cout << "overflow" << endl;
		else cout << ans << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <queue>
#include <stack>
#include <vector>
#include <stdio.h>
#include <algorithm>
#include <string.h>
#include <string>
#include <cmath>
#include <complex>
#include <map>
using namespace std;

#define rep(i,n) for(int i=0;i<int(n);++i)
#define ALL(v) (v).begin(),(v).end()
#define PB push_back
#define EPS 1e-8
#define F first
#define S second

static const double PI=6*asin(0.5);
typedef pair<double,double> P;
typedef complex<double> CP;
typedef long long ll;


int a[100],b[100],c[100];
char t;
string s;

int main(){
	int n;
	cin>>n;
	rep(i,n){
		memset(a,0,sizeof(a)); memset(b,0,sizeof(b));
		memset(c,0,sizeof(c));
		cin>>s;
		//cout<<s<<endl;
		if(s.size()>80){
			cout<<"overflow\n";
			continue;
		}
		rep(j,s.size()){
			a[90-j]=s[s.size()-j-1]-'0';
			//cout<<s[s.size()-j-1]-'0';
		}
		//cout<<a[87]<<endl;
		cin>>s;
		if(s.size()>80){
			cout<<"overflow\n";
			continue;
		}
		rep(j,s.size()){
			b[90-j]=s[s.size()-j-1]-'0';
		}
		/*
		rep(j,91) cout<<a[j];
		cout<<endl;
		rep(j,91) cout<<b[j];
		cout<<endl;
		*/
		int t=0;
		for(int j=90;j>=0;j--){
			c[j]=(a[j]+b[j]+t)%10;
			if((a[j]+b[j]+t)>9) t=1;
			else t=0;
		}
		bool f=true;
		/*
		rep(j,91) cout<<c[j];
		cout<<endl;
		*/
		rep(j,91){
			if(f&&c[j]!=0){
				f=false;
				if(j<=10){
					cout<<"overflow";
					break;
				}
			}
			if(!f) cout<<c[j];
		}
		cout<<endl;
		
	}
	return 0;
}

C++: #include <cstdio>
#include <cstdlib>
#include <cmath>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <sstream>
#include <queue>
#include <cstring>
#include <functional>

using namespace std;

#define REP(i,a,b) for((i)=(a);(i)<(int)(b);(i)++)
#define rep(i,n) REP(i,0,n)
// BEGIN CUT HERE
#define foreach(itr,c) for(typeof((c).begin()) itr=(c).begin();itr!=(c).end();itr++)
template<class T> inline string toString(T x){ostringstream sout;sout<<x;return sout.str();}
// END CUT HERE

int main(){
  int i, j, n;
  int len1, len2;
  char tmp;

  scanf("%d",&n);
  rep(i,n){
    string str1, str2, res;
    cin >> str1;
    cin >> str2;

    vector<int> a(200, 0), b(200, 0);
    len1 = (int)str1.size(); len2 = (int)str2.size();
    rep(j,len1) a[j] = str1[(int)len1-1-j] - '0';
    rep(j,len2) b[j] = str2[(int)len2-1-j] - '0';

    rep(j,200){
      int sum = a[j] + b[j];
      if(sum >= 10){ sum %= 10; a[j+1]++;}
      res += (sum + '0');
    }

    reverse(res.begin(), res.end());
    int f = 0;
    rep(j,res.size()){
      if(res[j] != '0') f = 1;
      if(f && res.size()-j > 79){
        cout << "overflow" << endl;
        return 0;
      }
      if(f) cout << res[j];
    }

    cout << endl;
  }

  return 0;
}

C++: #include <cstdio>
#include <vector>
using namespace std;

const int size = 80;
char str1[size + 10], str2[size + 10];
int num1[size], num2[size], res[size];
int n, leng1, leng2, rep, kuri;
bool output;

int main(){
	scanf("%d", &n);
	
	for(int a = 0; a < n; a++){
		fill(str1, str1 + size + 10, 0);
		fill(str2, str2 + size + 10, 0);
		fill(num1, num1 + size, 0);
		fill(num2, num2 + size, 0);
		fill(res, res + size, 0);
		scanf("%s", str1);
		scanf("%s", str2);
		
		//char -> int
		for(leng1 = 0; (int)str1[leng1 + 1] >= 48 && (int)str1[leng1 + 1] <= 57 && leng1 + 1 < size + 10; leng1++);//length of str1
		for(leng2 = 0; (int)str2[leng2 + 1] >= 48 && (int)str2[leng2 + 1] <= 57 && leng2 + 1 < size + 10; leng2++);//length of str2
		
		rep = 1;
		while(leng1 >= 0){
			num1[size - rep] = (int)str1[leng1] - 48;
			leng1--; rep++;
		}
		rep = 1;
		while(leng2 >= 0){
			num2[size - rep] = (int)str2[leng2] - 48;
			leng2--; rep++;
		}
		kuri = 0;
		for(int i = size - 1; i >= 0; i--){
			res[i] = num1[i] + num2[i] + kuri;
			kuri = res[i] / 10;
			if(kuri > 0) res[i] -= 10;
		}
		
		if(kuri > 0) printf("overflow\n");
		else{
			output = false;
			for(int i = 0; i < size; i++){
				if(!output && res[i] == 0) continue;
				else{
					output = true;
					printf("%d", res[i]);
				}
			}
			if(!output) printf("0");
			printf("\n");
		}
	}
	return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include <algorithm>
#include<string>
#include <utility>
using namespace std;
using namespace std;
int main(){
	int m;
	string in,in2;
	int a[90],b[90],ans[90];
	int f;
	int a_len,b_len,m_len;
	cin>>m;
	while(m--){
		for(int i=0;i<90;i++){
			a[i]=b[i]=ans[i]=0;
		}
		cin>>in;
		cin>>in2;
		a_len=in.size();
		b_len=in2.size();
		for(int i=0;i<a_len;i++){
			a[a_len-1-i]=in[i]-'0';
		}
		for(int i=0;i<b_len;i++){
			b[b_len-1-i]=in2[i]-'0';
		}
		m_len=max(a_len,b_len);
		if(m_len>80){
			cout<<"overflow"<<endl;
			continue;
		}
		f=0;
		for(int i=0;i<m_len+1;i++){
			ans[i]=(a[i]+b[i]+f)%10;
			f=(a[i]+b[i]+f)/10;
		}
		if(ans[m_len]!=0){
			cout<<ans[m_len]<<ans[m_len-1];
		}
		
		
	}
	
	
	return 0;
}