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 <stdio.h>
#include <iostream>
#include <string>

using namespace std;

int main() {

	string a;
	string b;
	int kari;

	cin >> kari;

	for (int o = 0; o < kari; o++) {
		int c[80] = {0};
		int d[80] = {0};
		int e[81] = {0};
		int counter;

		cin >> a;
		cin >> b;
	
		if (a.size() > 80 || b.size() > 80) {
			cout << "overflow" << endl;
		} else {
			for (int i = a.size() - 1; i >= 0; i--) {
				c[a.size() - 1 - i] = a[i] - '0';
			}

			for (int i = b.size() - 1; i >= 0; i--) {
				d[b.size() - 1 - i] = b[i] - '0';
			}

			for (int j = 0; j < 80; j++) {
				e[j] += c[j] + d[j];
				if (e[j] >= 10) {
					e[j+1] = e[j] / 10;
					e[j] = e[j] % 10;
				} 
			}
		}

		if (e[80] > 0) {
			cout << "overflow" << endl;
		} else {
			for (int k = 79; k >= 0; k--) {
				if (e[k] != 0) {
					counter = k;
					break;
				}
			}
			for (int k = counter; k >= 0; k--) {
				cout << e[k];
			}

			cout << endl;
		}
	}
	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;

int main(){
	int N;
	cin >> N;
	while(N--){
		string a,b;
		cin >> a >> b;
		reverse(a.begin(),a.end());
		reverse(b.begin(),b.end());
		while(a.size()>b.size())b += '0';
		while(a.size()<b.size())a += '0';
		string r;
		int c = 0; 
			for(int i = 0 ; i < a.size() ; i++){
			int d = a[i] + b[i] + c - 2 * '0';
			if( d >= 10 ) c = 1;
			r += d%10 + '0';
		}
		if(c)r += '0' + c;
		
		while(r.size()>=2 and r.back() == '0' )
			r = r.substr(0,r.size()-1);
		
		reverse(r.begin(),r.end());
		if( r.size() > 80 ) cout << "overflow" << endl;
		else cout << r << endl;
	}
}

C++: #include "bits/stdc++.h"

#define debug(x) cout<<#x<<": "<<x<<endl
#define rep(i,n) for (int i=0;i<(n);i++)
#define FOR(i,a,b) for (int i=(a);i<=(b);i++)
#define all(a) (a).begin(),(a).end()
using namespace std;
typedef vector<int> VI;
typedef vector<vector<int>> VVI;
typedef long long ll;
ll gcd(ll a, ll b) {
	ll c;
	while (a != 0) {
		c = a; a = b%a;  b = c;
	}
	return b;
}

void solve() {
#ifdef _WIN32
	istream &cin = ifstream("input.txt");
#endif
	int n;
	cin >> n;
	rep(i, n) {
		vector<ll> v1(11), v2(11);
		string s1, s2;
		cin >> s1 >> s2;
		rep(j, 10) {
			if (s1.size() > 10 * j) v1[j] = stoi(s1.substr(10 * j, (s1.size() - 10 * j >= 10 ? 10 : s1.size() - 10 * j)));
			if (s2.size() > 10 * j) v2[j] = stoi(s2.substr(10 * j, (s2.size() - 10 * j >= 10 ? 10 : s2.size() - 10 * j)));
		}
		rep(j, 11) {
			v1[j] += v2[j];
			if (v1[j] >= 1e10) {
				v1[j] -= 1e10;
				v2[j]++;
			}
		}
		if (v1[8] || v1[9] || v1[10]) cout << "overflow" << endl;
		else {
			rep(j, 11) if (v1[10 - j] != 0) cout << v1[10 - j];
			cout << endl;
		}
	}

}




int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);


	solve();
	//system("PAUSE");
	return 0;
}

C++: #include<stdio.h>
#include<iostream>
#include<string>
using namespace std;
int main(){

int n;
cin>>n;
for(int iii=0;iii<n;iii++){
string st1,st2;
int n1[83]={},n2[83]={};
cin>>st1>>st2;
for(int i=0;i<st1.size();i++)
n1[i]=st1[st1.size()-i-1]-'0';
for(int i=0;i<st2.size();i++)
n2[i]=st2[st2.size()-i-1]-'0';
int N[83]={};int y=0;

int r=st1.size();
if(r<st2.size())r=st2.size();
for(int i=0;i<r;i++){
N[i]=(n1[i]+n2[i]+y)%10;
y=(n1[i]+n2[i]+y)/10;
if(i==r-1&&y>0)r++;
}
if(r>80)cout<<"overflow"<<endl;
else{
for(int i=0;i<r;i++)
cout<<N[r-i-1];
cout<<endl;
}}
return 0;
}

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

#define rep2(x,from,to) for(int x=(from);(x)<(to);(x)++)
#define rep(x,to) rep2(x,0,to)
int main() {
	int n;
	cin >> n;
	while(n--) {
		char a[84], b[84];
		int x[84], y[84], s[84], max;
		cin >> a >> b;
		int i = 0, j = 0;
		for(; a[i] != '\0'; ++i) {
			x[i] = a[i] - '0';
		}
		for(; b[j] != '\0'; ++j) {
			y[j] = b[j] - '0';
		}
		if(i > j) {
			for(int k = j - 1; k >= 0; k--) {
				y[i-j+k] = y[k];
			}
			rep(k,i-j) {
				y[k] = 0;
			}
			max = i;
		} else if(i < j) {
			for(int k = i - 1; k >= 0; k--) {
				x[j-i+k] = x[k];
			}
			rep(k,j-i) {
				x[k] = 0;
			}
			max = j;
		}
		bool f = 0;
		for(int k = max - 1; k >= 0; k--) {
			int c = x[k] + y[k];
			if(f) c++;
			if(c >= 10) {
				f = 1, c -= 10;
			} else f = 0;
			s[k] = c;
		}
		if(f) {
			max++;
		}
		if(max > 80) cout << "overflow" << endl;
		else {
			if(f) {
				cout << "1";
				max--;
			}
			rep(k,max) {
				cout << s[k];
			}
			cout << endl;
		}
	}
	return 0;
}

C++: #include <iostream>
#include <vector>
#include <cstdio>
#define N 10000
using namespace std;

char s[N];
vector<int> a,b;
int n, an,bn;

void pr( vector<int> aa ){
  unsigned int i=0, len;
  while( aa[i]==0 ) i++;

  len = aa.size()-i;
  //  cout << "length : "<< len << endl;
  if( len>=80 ){
    cout << "overflow" << endl;
  }else{
    for( ;i<aa.size();i++ )
      cout << aa[i];
    cout << endl;
  }
}

int main(){
  cin >> n;
  fgets( s,N,stdin );
  for( int loop=0;loop<n;loop++ ){

    a.clear();
    fgets( s,N-1,stdin );    
    for( int i=0; s[i]!='\n' ;i++ )
      a.push_back( s[i]-'0' );

    b.clear();
    fgets( s,N,stdin );
    for( int i=0;s[i]!='\n';i++ )
      b.push_back( s[i]-'0' );

    an=a.size(); bn=b.size();
    while( an>bn ){
      b.insert( b.begin(),1, 0 ); bn++;
    }
    while( an<bn ){
      a.insert( a.begin(),1, 0 ); an++;
    }
    //    pr(a);
    //    pr(b);
    a.insert( a.begin(),1,0 );
    b.insert( b.begin(),1,0 );
    for( int i=an;i>=0;i-- ){
      a[i] = a[i] + b[i];
      while( a[i]>=10 ){
	a[i]-=10; a[i-1]++;
      }
    }
    pr( a );
  }
  return 0;
}

C++: #include <iostream>
#include <string>
#include <algorithm>
using namespace std;
int main(){
	int n,m;
	string aindex, bindex, ans;
	cin >> m;
	n = 0;
	string a,b;
	for(int j = 0; j < m; j++){
		cin >> a >> b;
		if(a.size() > 80||b.size() > 80){
			cout << "overflow" << endl;
		}else{
			for(int i = 0; i< max(a.size(),b.size()); i++){
				if(a.size()>= i+1){
					aindex = a[a.size()-i-1];
				}else{
					aindex = "0";
				}
				if(b.size() >= i+1){
					bindex = b[b.size()-i-1];
				}else{
					bindex = "0";
				}
				n = stoi(aindex) + stoi(bindex) + (n-n%10==10);
				aindex = to_string(n%10);
				ans = aindex + ans;
			}

			if(n-n%10 == 10){
				ans = '1' + ans;
			}

			if(ans.size() > 80){
				cout << "overflow" << endl;
			}else{
				cout << ans << endl;
			}

		}
		ans ="";
		n = 0;
	}
	return 0;
}