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 <cstring>
using namespace std;

int main(){
  int j;
  char s1[111] = {0};
  char tmp[111] = {0};
  char s2[111] = {0};
  char ans[111];
  int  kuri = 0 ;
  memset(ans,0,sizeof(ans));
  cin >> s1 >> s2;
  if(strlen(s1) < strlen(s2)){
      for(int i=0,j=strlen(s1)-1;i<=j;i++,j--) swap(s1[i],s1[j]);
      for(int i=0,j=strlen(s2)-1;i<=j;i++,j--) swap(s2[i],s2[j]);
      for(int i=0;i<strlen(s2);i++){
	if((s2[i] + (s1[i]%'0') + kuri) > '9'){
	  ans[i] = (s2[i] + (s1[i]%'0') + kuri)-10;
	  kuri = 1;
	}
	else {
	  ans[i] = (s2[i] + (s1[i]%'0') + kuri);
	  kuri = 0;
	}
	if(i == strlen(s2)-1 && kuri == 1) ans[i+1] = '1';
      }
      for(int i=strlen(ans)-1;i>=0;i--) cout << ans[i];
  }
  else{
    for(int i=0,j=strlen(s1)-1;i<=j;i++,j--) swap(s1[i],s1[j]);
    for(int i=0,j=strlen(s2)-1;i<=j;i++,j--) swap(s2[i],s2[j]);
    for(int i=0;i<strlen(s1);i++){
      if((s1[i] + (s2[i]%'0') + kuri) > '9'){
	ans[i] = (s1[i] + (s2[i]%'0') + kuri)-10;
	kuri = 1;
      }
      else {
	ans[i] = (s1[i] + (s2[i]%'0') + kuri);
	kuri = 0;
      }
      if(i == strlen(s1)-1 && kuri == 1) ans[i+1] = '1';
    }
    for(int i=strlen(ans)-1;i>=0;i--) cout << ans[i];
  }
  cout << endl;
}

C++: #include <iostream>
#include <stdio.h>
#include <sstream>
#include <string>
#include <vector>
#include <map>
#include <queue>
#include <algorithm>
#include <set>
#include <math.h>
#include <utility>
#include <stack>
using namespace std;
typedef pair<int,int> P;
const int INF = ~(1<<31) / 2;

int main(){
    int k;
    cin >> k;
    for(int j=0;j<k;j++){
        int n[80];
        string a,b;
        cin >> a >> b;
        int n_a = a.size(),n_b = b.size();
        if(n_a>80||n_b>80){cout << "overflow" << endl;continue;}

        fill(n,n+80,0);
        bool is_over = false;
        for(int i=0;i<max(n_a,n_b);i++){
            int ai,bi;
            if (i<n_a) ai = a[n_a-i-1]-'0';
            else ai = 0;
            if (i<n_b) bi = b[n_b-i-1]-'0';
            else bi = 0;
            n[i] += ai + bi;
            if (n[i]>=10){
                if(i+1 == 80){
                    is_over = true;
                    break;
                }else{
                    n[i+1] += n[i]/10;
                    n[i] = n[i]%10;
                }
            }
        }
        if (is_over){
            cout << "overflow" << endl;
        }else{
            bool started = false;
            for(int i=79;i>=0;i--){
                if(!started && n[i]>0){
                    started = true;
                }
                if(started){
                    cout << n[i];
                }
            }
            cout << endl;
        }
    }
    return 0;
}

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;
}