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 <algorithm>
using namespace std;

int main(){
	int n;
	cin>>n;
	for(int i=0; i<n; i++){
		string a, b;
		cin>>a>>b;
		int kuri = 0;
		int k = 0;
		string ans = "";
		while(true){
			bool breakflg = true;
			int ai = 0;
			int bi = 0;
			if(int(a.size())-k-1 >= 0){
				ai = a[int(a.size())-k-1] - '0';
				breakflg = false;
			}
			if(int(b.size())-k-1 >= 0){
				bi = b[int(b.size())-k-1] - '0';
				breakflg = false;
			}
			k++;
			int sum = ai + bi + kuri;
			if(!kuri&&breakflg) break;
			if(sum>9){
				kuri = 1;
				sum %= 10;
			} else {
				kuri = 0;
			}
			ans.push_back('0'+sum);
		}
		reverse(ans.begin(), ans.end());
		cout<<ans<<endl;
	}
	return 0;
}

C++: #include <stdio.h>

#define DEGITS 8

void load(int a[]);
void print(int a[]);

int main(void){
    int a[DEGITS]; // a[0] = LSD
    int b[DEGITS];
    int n;
    int cry;
    
    scanf("%d", &n);
    for (int i=0; i<n; i++){
        // reset
        for (int i=0; i<DEGITS; i++){
            a[i]=b[i]=0;
        }
        // load
        load(a);
        load(b);
        // add ( a <- a + b )
        cry=0;
        for (int d=0; d<DEGITS; d++){
            a[d] += (b[d] + cry);
            cry = a[d]/10;
            if (a[d]>9){
                a[d]-=10;
            }
        }
        // print
        if (cry){
            printf("overflow\n");
        } else{
            print(a);
        }
    }

    return 0;
}

void load(int a[]){
    char *pt;
    char st[DEGITS+1];
    
    scanf("%s", st);
    for (pt=st; *pt!='\0'; pt++) ;
    for (int i=0; pt!=st; i++){
        a[i] = *(--pt)-'0';
    }
}

void print(int a[]){
    int d;
    
    for (d=DEGITS-1; d>0; d--){ 
        if (a[d] != 0) break;
    }
    for (; d>0; d--) printf("%d", a[d]);
    printf("%d\n", a[0]);
}

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

#define NUMBER_DIGITS 80

int main()
{
  int N;
  char str1[NUMBER_DIGITS + 1];
  char str2[NUMBER_DIGITS + 1];
  char str3[NUMBER_DIGITS / 2 + 1][3];
  char str4[NUMBER_DIGITS / 2 + 1][3];
  int n1[NUMBER_DIGITS / 2 + 1] = {};
  int n2[NUMBER_DIGITS / 2 + 1] = {};
  cin >> N;
  for(int i = 0; i < N; i++){
    cin >> str1;
    cin >> str2;
    int m1 = strlen(str1), m2 = strlen(str2);
    if(m1 > NUMBER_DIGITS || m2 > NUMBER_DIGITS)
      cout << "overflow" << endl;
    else if(atoi(str1) + atoi(str2) < 2000000000){
      cout << atoi(str1) + atoi(str2) << endl;
      break;
    }else{
      for(int i = 2, j = NUMBER_DIGITS / 2; i <= m1 + 1; i += 2, j--){
	if(m1 % 2 == 1 && m1 - i < 0){
	  str3[j][0] = str1[0];
	  str3[j][1] == '\0';
	}else{
	  strncpy(str3[j], &str1[m1 - i], 2);
	  str3[j][2] == '\0';
	}
      }
      for(int i = 2, j = NUMBER_DIGITS / 2; i <= m2 + 1; i += 2, j--){
	if(m2 % 2 == 1 && m2 - i < 0){
	  str4[j][0] = str2[0];
	  str4[j][1] == '\0';
	}else{	  
	  strncpy(str4[j], &str2[m2 - i], 2);
	  str4[j][2] == '\0';
	}
      }
      for(int i = NUMBER_DIGITS / 2; i + ((m1 % 2 == 1) ? 1 : 0) >= NUMBER_DIGITS / 2 - m1 / 2; i--)
	if(str3[i][0] == '0' && str3[i][1] == '0')
	  n1[i] = 1000;
	else
	  n1[i] = atoi(str3[i]);
      for(int i = NUMBER_DIGITS / 2; i + ((m2 % 2 == 1) ? 1 : 0) >= NUMBER_DIGITS / 2 - m2 / 2; i--)
	if(str4[i][0] == '0' && str4[i][1] == '0')
	  n2[i] = 1000;
	else
	  n2[i] = atoi(str4[i]);
      int ans[NUMBER_DIGITS / 2 + 1] = {};
      int i = 0;
      int m = (m1 > m2) ? m1 : m2;
      for(int j = NUMBER_DIGITS / 2; NUMBER_DIGITS / 2 - 1 - m / 2 < ((m % 2) ? j + 1 : j); i++, j--)
	if(n1[j] + n2[j] >= 100)
	  if(n1[j] == 1000 && n2[j] < 10){
	    n1[j - 1] *= 10;
	    ans[i] = n2[j];
	  }else if(n2[j] == 1000 && n1[j] < 10){
	    n2[j - 1] *= 10;
	    ans[i] = n1[j];
	  }else if(n1[j] == 1000 && n2[j] == 1000){
	    if(n1[j - 1] % 10 != 0)
	      n1[j - 1] *= 10;
	    ans[i] = 1000;
	  }else if(n1[j] == 1000 && n2[j] >= 10){
	    ans[i] = n2[j];
	  }else if(n2[j] == 1000 && n1[j] >= 10){
	    ans[i] = n1[j];
	  }else if(n1[j] + n2[j] < 110){
	    n1[j - 1]++;
	    ans[i] = n1[j] + n2[j] + 900;
	  }else{
	    n1[j - 1]++;
	    ans[i] = n1[j] + n2[j] - 100;
	  }
	else
	  ans[i] = n1[j] + n2[j];
      int k = i;
      for(; i > 0; i--)
	if((i == k) && (ans[i - 1] == 0))
	  ;
	else if(ans[i - 1] == 1000)
	  cout << "00";
	else if(ans[i - 1] > 1000)
	  cout << "0" << ans[i - 1] % 1000;
	else
	  cout << ans[i - 1];
      cout << endl;
    }
  }
}

C++: #include <bits/stdc++.h>
using namespace std;
using i64 = long long;
#define endl "\n"

int main()
{
  i64 N;
  cin >> N;
  for (i64 _ = 0; _ < N; _++)
  {
    string a, b;
    cin >> a >> b;
    reverse(a.begin(), a.end());
    reverse(b.begin(), b.end());
    string ans;
    bool up = false;
    for (i64 i = 0; i < max(a.size(), b.size()); i++)
    {
      i64 n = (i < a.size() ? a[i] - '0' : 0) + (i < b.size() ? b[i] - '0' : 0) + (up);
      up = 10 <= n;
      ans += (char)((n % 10) + '0');
    }
    if (up)
      ans += "1";
    reverse(ans.begin(), ans.end());
    if (80 < ans.size())
      cout << "overflow" << endl;
    else
      cout << ans << endl;
  }
  return 0;
}

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

int charToInt(char);
string sumStr(string, string);

int charToInt(char c) {
  return (int)c - (int)'0';
}

string sumStr(string a, string b) {
  int pa = a.length()-1, pb = b.length()-1;
  string str(83, '0');
  int pstr = str.length()-1;
  int next = 0;
  while(pa >= 0 && pb >= 0) {
    int temp = charToInt(a[pa]) + charToInt(b[pb]) + next;
    next = temp / 10;
    str[pstr] = (char)(temp%10+(int)'0');

    pa--;
    pb--;
    pstr--;
  }
  while(pa >= 0) {
    int temp = charToInt(a[pa]) + next;
    next = temp / 10;
    str[pstr] = (char)(temp%10+(int)'0');

    pa--;
    pstr--;
  }
  while(pb >= 0) {
    int temp = charToInt(b[pb]) + next;
    next = temp / 10;
    str[pstr] = (char)(temp%10+(int)'0');

    pb--;
    pstr--;
  }
  str[pstr] = (char)(next + (int)'0');

  for(int i = 0; i < str.length(); i++) {
    if(str[i] != '0') {
      return str.substr(i, str.length()-i);
    }
  }
  return "0";
}

int main() {
  int N;
  cin >> N;
  for(int i = 0; i < N; i++) {
    bool overflow = false;
    int data[N][8];
    string sum;
    for(int j = 0; j < 2; j++) {
      string str;
      cin >> str;

      if(!overflow) {
	if(j == 0) sum = str;
	else       sum = sumStr(sum, str);
        if(sum.length() > 80) overflow = true;
      }
    }

    if(overflow) cout << "overflow" << endl;
    else         cout << sum << endl;
  }
  return 0;
}

C++: #define _CRT_SECURE_NO_WARNINGS
#define M_PI 3.14159265358979
#include <iostream>
#include <stdio.h>
#include <string>
#include <math.h>
#include<iomanip>
#include<vector>
#include<cmath>
#include <algorithm>
#include <cstdlib>
#include <typeinfo>
using namespace std;



int main(){
	int n;
	string x, y;
	int a, b, c;
	cin >> n;
	for (int i = 0; i < n; i++)
	{
		cin >> x >> y;
		if (x.length() > 80 || y.length() > 80)
		{
			cout << "overflow" << endl;
		}
		else
		{
			a = stoi(x);
			b = stoi(y);
			if (to_string(a + b).length() > 80)
			{
				cout << "overflow" << endl;
			}
			else cout << a + b << endl;
		}
	}
}