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 <cstdio>
#include <cstdlib>
#include <cstdint>
#include <cstring>
#include <cctype>
#include <iostream>
#include <sstream>
#include <vector>
#include <list>
#include <deque>
#include <map>
#include <set>
#include <stack>
#include <queue>
#include <unordered_map>
#include <unordered_set>
#include <algorithm>
#include <functional>
#include <stdexcept>
using namespace std;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef long long ll;
const double PI = 3.141592653589793238462643383279502884L;
const int LARGE_INT = 1000000007;
#define all(c) (c).begin(), (c).end()
#define rall(c) (c).rbegin(), (c).rend()
#define tr(c, i) for(auto i = (c).begin(); i != (c).end(); i++)
#define rtr(c, i) for(auto i = (c).rbegin(); i != (c).rend(); i++)
#define repfn(i,a,cont,next) \
  for(auto i = (a); [=](){return(cont);}(); i = [=](){return(next);}())
#define repby(i,a,b,s) repfn(i,a,i<b,i+s)
#define repab(i,a,b) repby(i,a,b,1)
#define rep(i,b) repab(i,0,b)
#define pb push_back
#define sz(c) int((c).size())

typedef vector<int> bignum;
const int max_digit = 80;

bignum operator+(const bignum& lhs, const bignum& rhs)
{
  int tmp, carry = 0;
  bignum res(max_digit, 0);
  for (int i = 0; i < max_digit; i++) {
    tmp = lhs[i] + rhs[i] + carry;
    if (tmp > 9) {
      carry = 1;
      res[i] = tmp % 10;
    } else {
      carry = 0;
      res[i] = tmp;
    }
  }
  if (carry)
    throw runtime_error("overflow");
  return res;
}

ostream& operator<<(ostream& os, const bignum& rhs)
{
  bool skip = true;
  rtr(rhs, i) {
    if (*i == 0 && skip)
      continue;

    skip = false;
    os << *i;
  }
  return os;
}

istream& operator>>(istream& is, bignum& rhs)
{
  string s;
  int i = 0;
  is >> s;
  rhs.assign(max_digit, 0);
  for (auto p = s.rbegin(); p != s.rend(); p++, i++) {
    rhs[i] = *p - '0';
  }
  if (sz(rhs) > max_digit)
    throw runtime_error("overflow");
  return is;
}

int main(int argc, char **argv)
{
  int n;
  bignum a, b;

  cin >> n;
  try {
    while (n--) {
      cin >> a >> b;
      cout << (a + b) << endl;
    }
  } catch (runtime_error& re) {
    cout << re.what() << endl;
  }
  return 0;
}

C++: #include<iostream>
#include<string>
#include<cstdio>
#include<stack>
#include<algorithm>
using namespace std;
string str1;
string str2;
stack<int> sta;

int main()
{
  int n;
  int i,j;
  int exc;//??°?????????
  int sum;

  cin>>n;
  for(i=0;i<n;i++){
    cin>>str1;
    cin>>str2;
    if(str1.size()>80 || str2.size()>80){
      cout<<"overflow"<<endl;
      continue;
    }
    exc=0;
    for(j=1;j<=min(str1.size(),str2.size());j++){
      sum=str1[str1.size()-j]-'0'+str2[str2.size()-j]-'0'+exc;
      exc=sum/10;
      sta.push(sum%10);
    }
    if( str1.size() < str2.size() ){
      while(j<=str2.size()){
	sum=str2[str2.size()-j]-'0'+exc;
	exc=sum/10;
	sta.push(sum%10);
	j++;
      }
    }else if( str1.size() > str2.size() ){
      while(j<=str1.size()){
	sum=str1[str1.size()-j]-'0'+exc;
	exc=sum/10;
	sta.push(sum%10);
	j++;
      }
    }
    if(max(str1.size(),str2.size())==80 && exc==1){
      cout<<"overflow"<<endl;
      continue;
    }
    if(exc)
      cout<<exc;
    for(j=0;j<max(str1.size(),str2.size());j++){  
      cout<<sta.top();
      sta.pop();
    }
    cout<<endl;
  }
}

C++: #include <iostream>
#include <string>
#include <algorithm>
#define EACH(i,c) for(typeof((c).begin()) i=(c).begin(); i!=(c).end(); ++i)
using namespace std;

int main(void){
	int N;
	cin >> N;
	for(int n=0;n<N;n++){
		string s1,s2,s3;
		cin >> s1 >> s2;
		reverse(s1.begin(),s1.end());
		reverse(s2.begin(),s2.end());
		bool up=false;
		for(int i=0;i<s1.size()||i<s2.size();i++){
			if(i<s1.size()&&i<s2.size()){
				int sum=0;
				if(up)sum++;
				sum+=(s1[i]+s2[i]-'0'-'0');
				s3+=(sum%10)+'0';
				if(sum>=10)
					up=true;
				else
					up=false;
			}else if(i<s1.size()){
				int sum=0;
				if(up)sum++;
				sum+=(s1[i]-'0');
				s3+=(sum%10)+'0';
				if(sum>=10)
					up=true;
				else
					up=false;
			}else if(i<s2.size()){
				int sum=0;
				if(up)sum++;
				sum+=(s2[i]-'0');
				s3+=(sum%10)+'0';
				if(sum>=10)
					up=true;
				else
					up=false;
			}
		}
		if(up)
			s3+='1';
		if(s3.size()>80)
			cout << "overflow" << endl;
		else{
			reverse(s3.begin(),s3.end());
			cout << s3 << endl;
		}
	}
	return 0;
}

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


#define MAX 1000


using namespace std;


int main()
{
	int n = 0;
	string in[2];
	char out[MAX] = {NULL};

	cin >> n;

	for (int i = 0; i < n; i++){
		cin >> in[0] >> in[1];

		for (int i = 0; i < MAX; i++){
			out[i] = '\0';
		}

		int count = in[0].size() - 1;

		for (int i = 0; i < in[0].size(); i++){
			out[count--] += in[0][i];
		}

		count = in[0].size();
		int count2 = in[1].size() - 1;

		count = max(count , count2) - 1;

		for (int i = 0; i < in[1].size(); i++){
			if (out[i] != '\0'){
				out[i] -= '0';
			}
			out[i] += in[1][count2--];
			count--;
		}
		
		int i = 0;
		for (i = 0; out[i] != '\0'; i++){
			;
		}

		bool flag = false;


		for (int j = 0; j < i; j++){
			if (out[j] > '9'){
				out[j] -= 10;
				out[j + 1] += 1;
			}
		}

		if (out[i] > '\0' && out[i] < '0'){
			out[i] += '0';
		}
		
		for (i = 0; i < out[i] != '\0'; i++){
			;
		}

		for (int j = 0; j < i / 2; j++){
			char tmp = out[j];
			out[j] = out[i - j - 1];
			out[i - j - 1] = tmp;
		}

		for (i = 0; out[i]; i++){
			;
		}

		if (i >= 80 || (flag == true && i >= 79)){
			cout << "overflow" << endl;
		}else{
			cout << out << endl;
		}
	}

	return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include <algorithm>
#include <utility>
using namespace std;
using namespace std;
int main(){
	
	int m;
	char in[2][10000];
	char ans[100000];
	int ie[2];
	int ien[2];
	bool max10;
	int Min;
	bool NMin;
	scanf("%d",&m);
	for(;m--;){
		
		for(int i=0;i<2;i++){
			scanf(" %s",&in[i]);
		}
		for(int i=0;i<2;i++){
			for(ie[i]=0;in[i][ie[i]]!='\0';ie[i]++){
				in[i][ie[i]]=in[i][ie[i]]-'0';
			}
		}
		ie[0]--;
		ie[1]--;
		Min=max(ie[0],ie[1]);
		if((Min>80)||(Min>79)&&(in[0][0]+in[1][0]>9)){
			cout<<"overflow"<<endl;
		}else{	
			if(ie[0]>ie[1]){
				NMin=0;
			}else{
				NMin=1;
			}
			ien[0]=ie[0];
			ien[1]=ie[1];
			for(;min(ien[0],ien[1])+1;ien[0]--,ien[1]--){
				in[NMin][ien[NMin]]=in[0][ien[0]]+in[1][ien[1]];
				if(in[NMin][ien[NMin]+1]>9){
					in[NMin][ien[NMin]+1]-=10;
					in[NMin][ien[NMin]]  +=1;
				}
				
			}
			max10=0;
			if(in[NMin][ien[NMin]+1]>9){
				in[NMin][ien[NMin]+1]-=10;
				max10=1;
			}
			for(int i=0;i<2;i++){
				for(int j=0;j<ie[i]+1;j++){
					in[i][j]=in[i][j]+'0';
				}
			}
			if(max10) cout<<"1";
			cout<<in[NMin]<<endl;
		}
	}
	return 0;
}