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>
using namespace std;

#define MAX_SIZE 80

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

    string n1, n2;

    while(cin >> n1 >> n2)
    {

        if( n1.size()>80 || n2.size()>80 ){
                cout << "overflow" << endl;
                continue;
        }

        // 先頭を0で埋めて80桁に合わせる
        char num1[MAX_SIZE], num2[MAX_SIZE];
        for(int i=n1.size(); i<80; i++) n1.insert(0, "0");
        for(int i=n2.size(); i<80; i++) n2.insert(0, "0");

        int c=0; // carry out
        char s=0; // sum
        for(int i=0; i<MAX_SIZE; i++){
            s = n1.at(n1.size()-i-1) + n2.at(n2.size()-i-1) - 2*'0' + c;
            // 桁上がりの有無
            if(s>=10){ 
                c = 1;
                s -= 10;
            }else{
                c = 0;
            }
            n1.at(n1.size()-i-1) = s + '0';
        }
 
        if(c){ 
            cout << "overflow" << endl; 
        }else{
            while(n1.at(0)=='0') n1.erase(0,1); // 先頭の0を除く処理
            cout << n1 << endl;
        }

        n1.clear();
        n2.clear();
    }

    return 0;
}

C++: #include <algorithm>
#include <cctype>
#include <cstdio>
#include <cmath>
#include <iostream>
#include <functional>
#include <string>
#include <queue>
#include <vector>

using namespace std;

int main() {
  int N;
  cin >> N;
  for (int i = 0; i < N; i++) {
    int a[80], b[80], r[81];
    fill(a, a+80, 0);
    fill(b, b+80, 0);
    fill(r, r+81, 0);
    string sa, sb;
    cin >> sa >> sb;
    reverse(sa.begin(), sa.end());
    reverse(sb.begin(), sb.end());
    for (unsigned int j = 0; j < sa.length(); j++)
      a[j] = sa[j] - '0';
    for (unsigned int j = 0; j < sb.length(); j++)
      b[j] = sb[j] - '0';
    for (int j = 0; j < 80; j++) {
      r[j] += a[j] + b[j];
      r[j+1] += r[j] / 10;
      r[j] %= 10;
    }
    if (r[80]) {
      cout << "overflow" << endl;
      continue;
    }
    int k = 79;
    for (; k >= 1 && !r[k]; k--);
    for (int j = k; j >= 0; j--)
      cout << r[j];
    cout << endl;
  }
  return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
char str[102],str1[102],ans[102];
int sam(int ma1,int ma2)
{
    int co,carry=0,a,b;
    for(int i=0;ma1-i>=0||ma2-i>=0||carry==1;i++){
            if(ma1-i>=0&&ma2-i>=0){
                    a=str[ma1-i]-'0';
                    b=str1[ma2-i]-'0';
                    if(a+b+carry>=10){
                        ans[i]=a+b+carry-10+'0';
                        carry=1;
                    }
                    else {
                        ans[i]=a+b+carry+'0';
                        carry=0;
                    }
            }
            else if(ma1-i>=0){
                    a=str[ma1-i]-'0';
                    if(a+carry>=10){
                        ans[i]=a+carry-10+'0';
                        carry=1;
                    }
                    else {
                        ans[i]=a+carry+'0';
                        carry=0;
                    }
            }
            else if(ma2-i>=0){
                    b=str1[ma2-i]-'0';
                    if(b+carry>=10){
                        ans[i]=b+carry-10+'0';
                        carry=1;
                    }
                    else {
                        ans[i]=b+carry+'0';
                        carry=0;
                    }
            }
            else {
                ans[i]='0'+1;
                return i;
            }
        co=i;

    }
    return co;

}
int main()
{
    int a,b,n;
    cin>>n;
    for(int i=0;i<n;i++){
        cin>>str>>str1;
        a=strlen(str);
        b=strlen(str1);
        a=sam(a,b);
        if(a>80)cout<<"overflow"<<endl;
        else for(int j=a;j>0;j--)cout<<ans[j];
        cout<<endl;
    }

}

C++: #include <iostream>
#include <vector>
#include <string>
#include <sstream>
using namespace std;
string toString(int n) {
    ostringstream os;
    os << n;
    return os.str();
}
string add(string a, string b) {
    if (a.size() > 80 || b.size() > 80) {
        return "overflow";
    }

    int siz = max(a.size(), b.size());
    vector<int> s1(siz, 0);
    for (int i=a.size()-1; i>=0; --i) {
        s1[a.size()-1-i] = a[i] - '0';
    }
    vector<int> s2(siz, 0);
    for (int i=b.size()-1; i>=0; --i) {
        s2[b.size()-1-i] = b[i] - '0';
    }

    string res = "";
    int carry = 0;
    for (int i=0; i<siz; ++i) {
        res = toString((s1[i] + s2[i] + carry) % 10) + res;
        carry = (s1[i] + s2[i] + carry >= 10);
    }
    if (carry) {
        res = '1' + res;
    }

    if (res.size() > 80) {
        return "overflow";
    }
    return res;
}
int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n;
    cin >> n;
    for (int i=0; i<n; ++i) {
        string a, b;
        cin >> a >> b;
        cout << add(a, b) << endl;
    }
    return 0;
}

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

int main() {
  
  int n,i,j;
  cin >> n;

  for(i=0;i<n;i++){
    string str1,str2,temp;
    int kuriagari=0;
    cin >> str1 >> str2;
    
    if (str1.size()<str2.size()){
      temp = str1;
      str1 = str2;
      str2 = temp;
    }
    int goukei[str1.size()+1];
    
    for(j=0;j<str1.size()+1;j++){
      
      if (j==str1.size()){
	goukei[0] = kuriagari;
      }else if (j<str2.size()){
	goukei[str1.size()-j] = str2[str2.size()-1-j] + str1[str1.size()-1-j] + kuriagari -'0' - '0';
	if (goukei[str1.size()-j] >= 10) {
	  goukei[str1.size()-j] -= 10;
	  kuriagari = 1;
	}else{
	  kuriagari = 0;
	}
      }else{
	goukei[str1.size()-j] = str1[str1.size()-1-j] + kuriagari -'0';
	if (goukei[str1.size()-j] >= 10) {
	  goukei[str1.size()-j] -= 10;
	  kuriagari = 1;
	}else{
	  kuriagari = 0;
	}
      }
    }
    if (str.size()>81)||( (str.size()==80)&&(goukei[0]==1) ){
      cout << "overflow" <<endl;
    }else{
      if (goukei[0]!=0)
	cout << goukei[0] ;
      for(j=0;j<str1.size();j++)
	cout << goukei[j+1];
      cout <<endl;
    }
  }
      
  return 0;
  
}

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=0 ; l<a.size()-1 ; l++){
						a[a.size()-2-l] ++;
						if(a[a.size()-2-l] > '9'){
							if(a.size()-2-l == 0){
								/*string A;
								A += '0';
								A += a;
								a = A;*/
								a = "0" + a;
							}
							a[a.size()-2-l] -= 10;
						}else break;
					}
				}
			}
		}
		cout << a << endl;

		}

	return 0;
}

C++: #include <iostream>
#include <cmath>
#include <cstdlib>

using namespace std;

int main(){
	int N; cin >> N;
	for(int i = 0; i < N; i++){
		long double a, b;
		cin >> a >> b;
		if(log10(a + b) > 80) cout << "overflow" << endl, 
		exit(2);
		cout << a + b << endl;
	}
	return 0;
}