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>
#include <cstring>
using namespace std;

int main()
{
  int N;
  char str1[81];
  char str2[81];
  int j = 0, k = 0, c;
  cin >> N;
  for(int i = 0; i < N; i++){
    cin >> str1;
    cin >> str2;
    if(j > 80 || k > 80)
      cout << "overflow" << endl;
    else{
      int n1[41] = {};
      int n2[41] = {};
      int m1 = strlen(str1), m2 = strlen(str2);
      for(int i = 1, j = 40; (m1 - i - 1 >= 0) || (m1 - i >= 0); i += 2, j--)
	if(str1[m1 - i - 1] == '0')
	  if(str1[m1 - i] == '0')
	    n1[j] = 1000;
	  else 
	    n1[j] = str1[m1 - i] - '0';
	else
	  if(m1 - i - 1 < 0)
	    n1[j] = str1[m1 - i] - '0';
	  else
	    n1[j] = (str1[m1 - i] - '0') + (str1[m1 - i - 1] - '0') * 10; 
      for(int i = 1, j = 40; (m2 - i - 1 >= 0) || (m2 - i >= 0); i += 2, j--)
	if(str2[m2 - i - 1] == '0')
	  if(str2[m2 - i] == '0')
	    n2[j] = 1000;
	  else
	    n2[j] = (str2[m2 - i] - '0');
	else
	  if(m2 - i - 1 < 0)
	    n2[j] = str2[m2 - i] - '0';
	  else
	    n2[j] = (str2[m2 - i] - '0') + (str2[m2 - i - 1] - '0') * 10;
      int ans[41] = {};
      int i = 0;
      int m = (m1 > m2) ? m1 : m2;
      for(int j = 40; 39 - 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 <iostream>
#include <stdio.h>
#include <string.h>
#include <math.h>
#include <algorithm>
#include <stdlib.h>

using namespace std;


int main(){

	int n;
	cin >> n;
	for(int i=0 ; i<n ; i++){
		string a, b;
		cin >> a;
		cin >> b;

		if(b.size() > a.size()){
			string temp;
			temp = a;
			a = b;
			b = temp;
		}

		if(a.size() > 80){
			cout << "overflow" << endl;
		}else{
			for(int j=0 ; j<b.size() ; j++){
				a[a.size()-1-j] = a[a.size()-1-j] + b[b.size()-1-j] -'0';
				if(a[a.size()-1-j] > '9'){
					a[a.size()-1-j] -= 10;
					for(int l=j ; l<a.size()-1 ; l++){
						a[a.size()-2-l] ++;
						if(a[a.size()-2-l] > '9'){
							if(a.size()-2-l == 0){
								a = "0" + a;
							}
							a[a.size()-2-l] -= 10;
						}else break;
					}
				}
			}
			if(a.size() > 80){
				cout << "overflow" << endl;
			}else{
				cout << a << endl;
			}
		}

		}

	return 0;
}

C++: #include<stdio.h>
#include<string.h>
int main(void){
   int n;
   scanf("%d",&n);
   while(n--){
      char a[100],b[100],c[100];
      int j,len;

      for(j=0;j<81;j++){
         a[j]=0;
         b[j]=0;
         c[j]='0';
      }
      scanf("%s%s",a,b);
      len=strlen(a);
      if(len<strlen(b))
         len=strlen(b);
      if(len>80){
         printf("overflow\n");
         continue;
      }
      /*for(j=0;j<len+1;j++){
         c[j]='0';9999999999999999999999999999999999999999999999999999999999999999999999999999999
      }*/
      for(j=0;j<strlen(a);j++){
         c[j]=a[strlen(a)-j-1];
      }
      for(j=0;j<strlen(b);j++){
         c[j]+=b[strlen(b)-j-1]-'0';
      }
      for(j=0;j<len;j++){
         if(c[j]>=58){
            if(j==len-1){
               c[j+1]='0';
               len++;
            }
            c[j]-=10;
            c[j+1]+=1;
         };
      }
      if(len>80){
         printf("overflow\n");
         continue;
      }
      else{
         for(j=len-1;j>=0;j--){
            printf("%c",c[j]);
         }
         puts("");
      }
   }
   return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <vector>
#include <algorithm>
#include <cstring>
#include <stack>

using namespace std;

int main (int argc, char * const argv[]) {
    // insert code here...
	int n;
    string a, b;
		
	cin>> n;	
	for(int i=0;i<n;i++){
		int x[81]={0}, y[81]={0};

		cin >> a;
		cin >> b;
		
		reverse(a.begin(), a.end());
		reverse(b.begin(), b.end());
		
		for(int j=0;j<a.size();j++){
			x[j]=a[j]-'0';
		}
		for(int j=0;j<b.size();j++){
			y[j]=b[j]-'0';
		}
		
		for(int j=0;j<80;j++){
			x[j]=x[j]+y[j];
			if(x[j]>=10){
				x[j+1]++;
				x[j]-=10;
			}
		}
		//cout << 0 << endl;
		
		if(x[80]>0){
			cout << "overflow" <<endl;
		}else{
			bool flag = 0;
			for(int j=79;j>=0;j--){
				if(x[j]==0&& flag==0){
					continue;
				}else{
					flag=1;
				}
				cout<<x[j];
			}
			cout<<endl;
		}
		
	}
	
	return 0;
}

C++: #include <iostream>
#include <vector>
#include <set>
#include <algorithm>
#include <string>
#include <sstream>
#include <iomanip>
 
#define INPUT_FROM_FILE (0)
#if INPUT_FROM_FILE
#include <fstream>
#endif

void str_to_vec(std::string str, std::vector<char> &v){
	v.clear();
	for(int i = str.size() - 1; i >= 0; i--){
		v.push_back(str[i] - 48);
	}
	return;
}

std::vector<char> sum(std::vector<char> &v0, std::vector<char> &v1){
	std::vector<char> ret;
	char carry = 0;

	if(v0.size() < v1.size()){
		std::swap(v0, v1);
	}
	for(unsigned int i = 0; i < v1.size() - v0.size(); i++){
		v1.push_back(0);
	}

	ret.resize(v1.size(), 0);

	for(unsigned int i = 0; i < ret.size(); i++){
		char tmp = v0[i] + v1[i] + carry;
		if(tmp >= 10){
			carry = 1;
			tmp -= 10;
		}
		else{
			carry = 0;
		}
		ret[i] = tmp;
	}
	if(carry == 1){
		ret.push_back(1);
	}

	return ret;
}

int main(int argc, char **argv){
	int n = 0;
	std::string buf;

#if INPUT_FROM_FILE
	std::ifstream ifs("test.txt");
#endif



#if INPUT_FROM_FILE
	std::getline(ifs, buf);
#else
	std::getline(std::cin, buf);
#endif
	n = std::stoi(buf);

	for(int i = 0; i < n; i++){
		std::vector<char> num0;
		std::vector<char> num1;
#if INPUT_FROM_FILE
		std::getline(ifs, buf);
#else
		std::getline(std::cin, buf);
#endif
		str_to_vec(buf, num0);
#if INPUT_FROM_FILE
		std::getline(ifs, buf);
#else
		std::getline(std::cin, buf);
#endif
		str_to_vec(buf, num1);

		if(num0.size() > 80 || num1.size() > 80){
			std::cout << "overflow" << std::endl;
		}
		else{
			std::vector<char> s = sum(num0, num1);

			if(s.size() > 80){
				std::cout << "overflow" << std::endl;
			}
			else{
				for(int i = s.size() - 1; i >= 0; i--){
					std::cout << (int)s[i];
				}
				std::cout << std::endl;
			}
		}
	}
	return 0;
}

C++: #include <iostream>
#include <string>
#include <stack>
#include <math.h>
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <vector>

using namespace std;


bool f(long long x) {
	if (log10(x) >= 80) return false;
	else return true;
}



int main()
{
	int n;
	cin >> n;
	long long a;
	long long b;
	while(cin>>a){
		cin >> b;
		if (f(a) && f(b) && f(a + b)) cout << a + b << endl;
		else cout << "overflow" << endl;
	}
	

}