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<bits/stdc++.h>
using namespace std;


string solve(string s1, string s2){

while(s1.size()<85){
s1 = '0' + s1;
}
while(s2.size()<85){
s2 = '0' + s2;
}
string ret = "";
int kuri = 0;
for(int i=0;i<80;i++){
int r = (int)(s1[s1.size()-1-i]-'0' + s2[s2.size()-1-i]-'0') + kuri;
kuri = r/10;
r = r%10;
ret = ((char)('0'+r)) + ret;
}
if (kuri>0) return "overflow";
else{
string ret2 = "0";
for(int i=0;i<ret.size();i++){
if (ret[i]!='0'){ret2 = ret.substr(i);break;}
}
return ret2;
}
}

int main(void){
int N;
cin >> N;
for(int i=0;i<N;i++){
string s1, s2;
cin >> s1 >> s2;
cout << solve(s1, s2) << endl;
}
return 0;
}

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

int main() {
	int n;
	cin >> n;
	for (int x = 0;x < n;x++) {
		string a, b;
		cin >> a >> b;
		int za = a.size();
		int zb = b.size();
		vector<int>v;

		for (int i = 0;i < max(za, zb);i++) {
			if (i < min(za, zb)) {
				int c = za - i - 1;
				int d = zb - i - 1;
				v.push_back((a[za - i - 1] - '0') + (b[zb - i - 1] - '0'));
			}
			else {
				if (za < zb)v.push_back(b[zb - i - 1] - '0');
				else v.push_back((a[za - i - 1]) - '0');
			}
		}
		int z = v.size();
		for (int i = 0;i < v.size();i++) {
			if (i != z- 1 && v[i] >= 10) {
				v[i] -= 10;
				v[i + 1]++;
			}
		}
		if (v.size() > 80)cout << "overflow";
		else if (v.size() == 80 && v[79] >= 10)cout << "overflow";
		else 	for (int i =z - 1;i > -1;i--)cout << v[i];
		cout << endl;
	}
	return 0;
}

C++: /*****include*****/
#include <iostream>
#include <fstream>

/*****デバッグ定義*****/
//#define DEBUG
/*****マクロ定義*****/

/*****名前空間*****/
using namespace std;

/*****グローバル変数置き場*****/

/*****その他関数置き場*****/

/*****main関数*****/
int main(){
	/*****ファイルオープン*****/
#ifdef DEBUG
	ofstream fout("output.txt");
	ifstream fin("input.txt");
	if(!fout || !fin){
		cout << "Can't open the file.\n";
		return;
	}
#endif
	/*****変数置き場*****/
	char number1[81];
	char number2[81];
	int sum[81];
	int digit1;
	int digit2;
	int digit_sum;
	int move_up;
	int i,j;
	int n;
	/*****処理部*****/
	cin >> n;
	for(int loop=0;loop<n;loop++){
		for(int a=0;a<81;a++){
			number1[a] = '0';
			number2[a] = '0';
		}
		digit1 = 0;
		digit2 = 0;
		cin >> number1;
		cin >> number2;
		while(number1[digit1] != '\0')	digit1++;
		while(number2[digit2] != '\0')	digit2++;
		move_up = 0;
		//cout << digit1 << " " << digit2 << endl;
		if(digit1 != digit2){
			if(digit1 > digit2){
				i = digit1;
				j = digit2;
				while(j>=0){
					number2[i] = number2[j];
					number2[j] = '0';
					i--;j--;
				}
			}else{
				i = digit1;
				j = digit2;
				while(i>=0){
					number1[j] = number1[i];
					number1[i] = '0';
					i--;j--;
				}
			}
		}
		/*for(int i=((digit1 > digit2)? digit1 : digit2);i>0;i--){
			cout << number1[i-1];
		}
		cout << endl;
		for(int i=((digit1 > digit2)? digit1 : digit2);i>0;i--){
			cout << number2[i-1];
		}
		cout << endl;*/
		//for(int i=0;i<4;i++)	cout << number2[i] << " " << atoi(&number2[i]) << endl;
		//for(int i=0;i<4;i++)	cout << number1[i] << " " << atoi(&number1[i]) << endl;
		digit_sum = 0;
			for(int i=((digit1 > digit2)? digit1 : digit2);i>0;i--){
		//cout << atoi(&number1[i-1]) << " " << atoi(&number2[i-1]) << endl;
			sum[digit_sum]
				= number1[i-1] - '0'
					+ number2[i-1] - '0'
			+ move_up;
			if(sum[digit_sum] >= 10){
				move_up = 1;
				sum[digit_sum] = sum[digit_sum] % 10;
			}else{
				move_up = 0;
			}
			digit_sum++;
		}
		if(move_up == 1){
			sum[digit_sum] = move_up;
			digit_sum++;
		}
		//cout << digit_sum << endl;
		if(digit_sum > 80)	cout << "overflow" << endl;
		else{
			for(int k=digit_sum-1;k>=0;k--){
				cout << sum[k];
			}
			cout << endl;
		}
	}
	/*****処理終了後*****/
#ifdef DEBUG
	fout.close();
	fin.close();
#endif
	return 0;
}

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

#define DIGIT  ( 80 )

using namespace std;

static
int N;

/** Application main entry point. */
int main (
  int     argc,
  char  * argv[ ]
  )
{
  cin >> N;

  while ( N-- )
  {
    string        A, B;
    int           C[ DIGIT + 1 ] = { 0 };
    const char  * p;

    cin >> A >> B;

    p = A.c_str ( );
    for ( size_t i = 0; i < A.length ( ); ++i )
    {
      C[ i ] = p[ A.length ( ) - i - 1 ] - '0';
    }

    p = B.c_str ( );
    for ( size_t i = 0; i < B.length ( ); ++i )
    {
      C[ i ]     += p[ B.length ( ) - i - 1 ] - '0';
    }

    for ( size_t i = 0; i < DIGIT; ++i )
    {
      C[ i + 1 ] += C[ i ] / 10;
      C[ i ]     %= 10;
    }

    if ( C[ DIGIT ] )
    {
      cout << "overflow";
    }
    else
    {
      int i = DIGIT;
      while ( !C[ i ] && i > 0 )
        --i;
      while ( i >= 0 )
      {
        cout << C[ i-- ];
      }
    }
    cout << endl;
  }

  return ( 0 );
}

C++: #include<iostream>
#include<algorithm>
using namespace std;
int n;
int main()
{
	cin>>n;
	for(;n--;)
	{
		string s,t,u="";cin>>s>>t;
		if(s.size()<t.size())swap(s,t);
		int a=0;
		for(int i=0;i<s.size();i++)
		{
			a+=s[s.size()-i-1]-48;
			if(i<t.size())a+=t[t.size()-i-1]-48;
			u+=a%10+48;
			a/=10;
		}
		if(a)u+=a+48;
		if(u.size()>80)
		{
			cout<<"overflow"<<endl;
		}
		else
		{
			for(int i=u.size();i--;)cout<<u[i];
			cout<<endl;
		}
	}
}

C++: //0015 National Budget
//80ÜÅÌ2ÂÌ®Ì«µZÊðoÍ¹æ
//1sÚÉf[^Zbgnª^¦çêé
//^¦çê½â»Ìaª80ð´¦½êAoverflowÆoÍµÄ­¾³¢

#include<iostream>
#include<string>
#include<cstring>
#include<algorithm>
using namespace std;


int main(void)
{
	int n;
	cin >> n;
	while(n--)
	{
		const int N = 80;
		int i;
		string a, b;
		//0ú»
		char c[N+2] = "";

		cin >> a >> b;

		//vZµâ·¢æ¤É½]
		reverse(a.begin(), a.end());
		reverse(b.begin(), b.end());

		int aSize, bSize;
		aSize = a.size();
		bSize = b.size();

		//Ü¸ÍAcÉ»ÌÜÜlðüêÄ¢­
		for(int i = 0; i < max( max(aSize, bSize), N); i++)
		{
			int s = 0;
			if(i < aSize)
				s += a[i] - '0';
			if(i < bSize)
				s += b[i] - '0';

			c[i] += s;
			c[i+1] += c[i]/10;
			c[i] %= 10;
		}

		//ã©ç©Ä¢ÁÄA0ÈOÌ¶ª»ê½ç¶»·é
		bool isAnswer = false;
		for(i = sizeof(c)-1; i >= 0; i--)
		{
			if(c[i])
				isAnswer = true;
			if(isAnswer)
				c[i] += '0';
		}
		//½]µÄ®¬
		reverse(c, c+strlen(c));

		//àµà80¶ÚªNULL¶Å êÎI[o[t[
		if(c[N])
			cout << "overflow" << endl;
		else
			cout << c << endl;
	}
	return 0;
}

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

int main()
{
	int n;
	cin >> n;

	for(int i=0;i<n;++i){
		string N1,N2;
		vector<int> num_line;
		cin >> N1 >> N2;

		int upper=0;
		int num;
		string longer  = N1.size() > N2.size() ? N1 : N2;
		string shorter = N1.size() < N2.size() ? N1 : N2;

		for(int i=0;i<longer.size();++i){
			if(i < shorter.size())
				num = longer[i]-'0' + shorter[i]-'0' + upper;
			else
				num = longer[i] - '0' + upper;
			
			if(num > 9){
				upper = 1;
				if(i != longer.size() -1)
					num -= 10;
			}else{
				upper = 0;
			}

			num_line.push_back(num);
		}

		for(int i = num_line.size()-1;i>=0;--i){
			cout << num_line[i] ;
		}

	}
	return 0;
}