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 <cmath>
#include <vector>
#include <string>


using namespace std;

//?????????10?????????????????????
vector<string> splitstring(string str, int n){
  vector<string> tmp;
  for(int i=0;i<str.length();i+=n){
    //    cout << "str.substr: " << str.substr(i, n) << endl;
    tmp.push_back(str.substr(i, n));
  }

  return tmp;
}


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

  for(int i=0;i<numDatasets;i++){
    vector<string> splittedStr1, splittedStr2;
    vector<int> splittedInt1, splittedInt2;
    string str1, str2;
    cin >> str1;
    
    splittedStr1 = splitstring(str1, 7);
    for(int i=0;i<splittedStr1.size();i++){
      splittedInt1.push_back(std::stoi(splittedStr1[i]));
      // cout << splittedInt1[i] << endl;
    }

    cin >> str2;

    splittedStr2 = splitstring(str2, 7);
    for(int i=0;i<splittedStr2.size();i++){
      splittedInt2.push_back(std::stoi(splittedStr2[i]));
      // cout << splittedInt2[i] << endl;
    }

    vector<int> calced(max(splittedInt1.size(), splittedInt2.size()), 0);
    
    for(int i=0;i<calced.size();i++){
      if(i < splittedInt1.size()){
	calced[i] += splittedInt1[i];
      }
      if(i < splittedInt2.size()){
	calced[i] += splittedInt2[i];
      }
    }

    //debug
    for(int i=0;i<calced.size();i++){
      // cout << i << ": " <<  calced[i] << endl;
    }

    // cout << "---" << endl;

    
    //??°???????????????
    vector<int> calcedRes(calced.size()+1, 0);
    for(int i=0;i<calced.size();i++){
      int tmp = (calced[i]+calcedRes[i])/(int)pow(10.0, 7);
      calcedRes[i] = (calced[i]+calcedRes[i]) - tmp*pow(10.0, 7);
      calcedRes[i+1] = tmp;
      // cout << "calcedRes[" << i  << "]: " << calcedRes[i] << ", tmp: " << tmp << endl;
    }
      
      
      
      // calcedRes[calced.size()-i-2] += tmp;
      // calced[calced.size()-i-1] -= tmp*pow(10.0, 7);

      // int tmp = calced[i-1]/(int)pow(10.0, 7);
      // calced[i-1] -= tmp;
      // calced[i] += tmp; 

    // cout << "---" << endl;

    //int to str;
    string res = "";
    for(int i=calcedRes.size()-1;i>-1;i--){
      string str = to_string(calcedRes[i]);
      // cout << i << ":" << str << endl;
      if(i==calcedRes.size()-1){
	
	if(calcedRes[i]==0){
	  continue;
	}
      }

      while(str.length()<7){
	if(i==calcedRes.size()-1){
	  break;
	}
	str += "0";
      }


      
      res += str;
    }

    // cout << "result: " << res << endl;
    cout << res << endl;
  }
}

C++: #include<iostream>
#include<stdio.h>
#include<string>
#include<vector>
#include<string.h>
#include<stdlib.h>
#include<math.h>

using namespace std;

int main(){
	int n;
	string ret1,ret2;
	vector<int> result;
	int num = 0, ret = 0;
	int j;

	cin >> n;

	for (int i = 0;i < n;i++){

		ret = 0;

		cin >> ret1 >> ret2;

		if (ret1.size() > 80 || ret2.size() > 80){
			cout << "overflow" << endl;
		}

		for (j = 1;j <= ret1.size() && j <= ret2.size();j++){
			num = (ret1.at(ret1.size() - j) - '0' + ret2.at(ret2.size() - j) - '0' + ret) % 10;
			ret = (ret1.at(ret1.size() - j) - '0' + ret2.at(ret2.size() - j) - '0' + ret);
			ret = (ret - num)/10;
			result.push_back(num);
		}

		if (ret1.size() == ret2.size()){
			if (ret != 0)
				result.push_back(ret);
		}
		else if(ret1.size() > ret2.size()){
			for (; j <= ret1.size(); j++){
				num = (ret1.at(ret1.size() - j) - '0' + ret) % 10;
				ret = ret1.at(ret1.size() - j) - '0' + ret;
				ret = (ret - num) / 10;
				result.push_back(num);
			}
			if (ret != 0)
				result.push_back(ret);
		}
		else if (ret1.size() < ret2.size()){
			for (; j <= ret2.size(); j++){
				num = (ret2.at(ret2.size() - j) - '0' + ret) % 10;
				ret = ret2.at(ret2.size() - j) - '0' + ret;
				ret = (ret - num) / 10;
				result.push_back(num);
			}
			if (ret != 0)
				result.push_back(ret);
		}


		while (result.empty() != true){
			cout << result[result.size()-1];
			result.pop_back();
		}
		cout << endl;

	}


	return 0;
}

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

int main(){
  int j;
  int n;
  char s1[111];
  char tmp[111];
  char s2[111];
  char ans[180];
  int  kuri = 0;
  cin >> n;
  while(n--){
  memset(ans,0,sizeof(ans));
  memset(s1,0,sizeof(s1));
  memset(s2,0,sizeof(s2));
  memset(tmp,0,sizeof(tmp));
  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';
      }
      if(strlen(ans) > 80) cout << "overflow" << endl;
      else{
	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';
    }
    if(strlen(ans) > 80) cout << "overflow" << endl;
    else{
      for(int i=strlen(ans)-1;i>=0;i--) cout << ans[i];
    }
  }
  cout << endl;
  }
}

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

int main()
{
    int N,j;
    cin >> N;
    for(int i=0 ; i<N ; i++){
        int ans[81]={0};
        ans[0]=11111;
        char strA[81],strB[81];
        cin >> strA >> strB;
        if(strlen(strA)>80||strlen(strB)>80){
            cout << "overflow" << endl;
            break;
        }
        char str1[strlen(strA)][2],str2[strlen(strB)][2];
        for(int i=0 ; i<strlen(strA) ; i++){
            str1[i][0]=strA[i];
            str1[i][1]='\0';
        }
        for(int i=0 ; i<strlen(strB) ; i++){
            str2[i][0]=strB[i];
            str2[i][1]='\0';
        }
        int a[81]={0},b[81]={0};
        a[0]=11111;
        b[0]=11111;
        j=strlen(strA)-1;
        for(int i=80 ; j>=0 ; i--){
            a[i]=atoi(str1[j]);
            j--;
        }
        j=strlen(strB)-1;
        for(int i=80 ; j>=0 ; i--){
            b[i]=atoi(str2[j]);
            j--;
        }
        for(int i=80 ; i!=0 ; i--){
            ans[i]=ans[i]+a[i]+b[i];
            if(ans[i]>=10){
                ans[i]=ans[i]-10;
                ans[i-1]++;
            }
        }
        if(ans[0]!=11111){
            cout << "overflow" << endl;
            break;
        }
        for(int i=1 ; i<81 ; i++){
            if(ans[i]!=0){
                j=i;
                break;
            }
        }
        for(int i=j ; i<81 ; i++){
         cout << ans[i];
        }
        cout << '\n';
    }
    return 0;
}