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;

int reverse(string &s){
	int n = s.size(), n2 = n/2;
	for (int i=0; i<n2; i++){
		swap(s[i], s[n-1-i]);
	}
	return 0;
}

int main(void){
	int n;
	cin>>n;
	for (int i=0; i<n; i++){
		string s1, s2, s3="";
		cin>>s1>>s2;

		reverse(s1);
		reverse(s2);

		while (s1.size()<s2.size()) s1 += '0';
		while (s2.size()<s1.size()) s2 += '0';

		long c=0, stmp;
		for (int i=0; i<s1.size(); i++){
			stmp = s1[i]-'0'+s2[i]-'0'+c;
			c = stmp / 10;
			s3 += stmp%10 +'0';
		}
		if (c!=0) s3 += c + '0';

		reverse(s3);
		if (s3.size()>80)
			cout<<"overflow"<<endl;
		else
			cout<<s3<<endl;

	}
	return 0;
}

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

int main()
{
	string s1, s2, sum;
	int n;
	cin >> n;
	while (n--) {
		cin >> s1 >> s2;
		int len = max(s1.size(), s2.size());
		for (int i = 0; i < len; i++) {
			sum += "0";
		}
		// ?????????????????????????????????????????????'0'??§??°????????????
		if (s1.size() < s2.size()) {
			// s1????????????????????§s2.size()-s1.size()?????????'0'?????°????????????
			// ??????s1??????????????????
			string r_s1 = "";
			for (int i = s1.size() - 1; i >= 0; i--) {
				r_s1 += s1[i];
			}
			// r_s1???s2.size()-s1.size()?????????'0'???????????°??????
			for (int i = 0; i < s2.size() - s1.size(); i++) {
				r_s1 += "0";
			}
			// r_s1????????????????????????
			s1 = "";
			for (int i = r_s1.size() - 1; i >= 0; i--) {
				s1 += r_s1[i];
			}
		}
		else {
			// ??????s2??????????????????
			string r_s2 = "";
			for (int i = s2.size() - 1; i >= 0; i--) {
				r_s2 += s2[i];
			}
			// s1.size()-s2.size()?????????'0'??§????????°??????
			for (int i = 0; i < s1.size() - s2.size(); i++) {
				r_s2 += "0";
			}
			// r_s2??????????????????
			s2 = "";
			for (int i = r_s2.size() - 1; i >= 0; i--) {
				s2 += r_s2[i];
			}
		}
		
		int carry = 0;
		char s;
		for (int i = 0; i < len; i++) {
			if (carry == 0) {
				s = s1[s1.size() - 1 - i] - '0' + s2[s2.size() - 1 - i] - '0' + '0';
				if (s - '0' >= 10) {
					carry = 1;
					sum[len - 1 - i] = int((s - '0') % 10) + '0';
				}
				else {
					sum[len - 1 - i] = s;
				}
			}
			else {// carry over
				s = s1[s1.size() - 1 - i] - '0' + s2[s2.size() - 1 - i] - '0' + '0' + 1;
				if (s - '0' >= 10) {
					carry = 1;
					sum[len - 1 - i] = int((s - '0') % 10) + '0';
				}
				else {
					carry = 0;
					sum[len - 1 - i] = s;
				}
			}
		}

		cout << (carry == 1 ? "1" : "");
		for (int i = 0; i < len; i++) {
			cout << sum[i] - '0';
		}
		cout << endl;
	}
    return 0;
}

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

int main() {
	int lop;
	for (cin >> lop; lop > 0; lop--) {
		string a; cin >> a;
		string b; cin >> b;
		string c, d;
		int dif = a.size() - b.size();
		bool kuri = 0;
		for (int lop = 0; lop < abs(dif); lop++) {
			d += '0';
		}
		if (dif > 0)
			b = d + b;
		if (dif < 0)
			a = d + a;
		for (int i = 0; i < a.size(); i++) {
			c += '\0';
		}
		for (int i = a.size()-1; i >= 0; i--) {
			c[i] = a[i] + b[i]+kuri-'0';
			//cout << "c["<<i<<"] = "<<c[i] << endl;
			kuri = 0;
			if (c[i] > '9') {
				c[i] -= 10;
				kuri = 1;
			}
			if (i == 0 && kuri == 1)
				c = '1' + c;
		}
		if (c.size() > 80)
			cout << "overflow\n";
		else
			cout << c << endl;
	}
	return 0;
}

C++: #include "bits/stdc++.h"
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
#define rep(i,n) for(ll i=0;i<(ll)(n);i++)
#define all(a)  (a).begin(),(a).end()
#define vi vector<int>
#define pb push_back
#define INF 999999999
//#define INF (1LL<<59)

string addString(string a,string b ){
	if(a.size()<b.size())swap(a,b);         //len(a)>len(b)???
	b = string(a.size()-b.size(),'0')+b;    //b????????????a??¨??????????????????????????????0??§?????????
	
	string ans = string(a.size()+1,'0');    //a?????????+1 ?????????0??§???????????????
	for(int i=(int)a.size()-1;i>=0;i--){    //?¶??????? & ??°????????????
		ans[i+1] += (a[i]-'0') + (b[i]-'0');
		if(ans[i+1]>='0'+10){
			ans[i] += 1;
			ans[i+1] -= 10;
		}
	}
	string ret = "";                        //??????????????°???0????¶????
	rep(i,ans.size()){
		if(ans[i]!='0'||i==ans.size()-1){
			ret = ans.substr(i,ans.size()-i);
			break;
		}
	}
	return ret;
}


int main(){
	int n;
	cin>>n;
	rep(i,n){
		string a,b;
		cin>>a>>b;
		cout<<addString(a,b)<<endl;
	}
}

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

int main() {
	int n;cin>>n;
	string a,b;for(int j=0;j<n;j++){
	cin>>a>>b;
	int d=0;
	if(a.length()<b.length())
	{
		int aa=a.length();
		for(int i=0;i<b.length()-aa;i++)a='0'+a;
	}
	else
	{
		int bb=b.length();
		for(int i=0;i<a.length()-bb;i++)b='0'+b;
	}
	string ans="";
	for(int i=a.length()-1;i>=0;i--)
	{
		int aa=a[i]-'0';
		int bb=b[i]-'0';
		aa+=bb+d;d=0;
		if(aa>=10)
		{
			aa-=10;
			d++;
		}
		ans=char(aa+'0')+ans;
	}
	if(d)ans=char(d+'0')+ans;
	while(ans[0]=='0')
	{
		ans=ans.substr(1,ans.length()-1);
	}
	if(ans.length()>=80)cout<<"overflow"<<endl;
	else cout<<ans<<endl;}
	return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <iomanip>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <bitset>
#include <stack>
#include <utility>
#include <numeric>
#include <algorithm>
#include <functional>
#include <cctype>
#include <complex>
#include <string>
#include <sstream>

using namespace std;

#define all(c) c.begin(),c.end()
#define rall(c) c.rbegin(),c.rend()
#define rep(i,n) for(int i=0;i<(n);i++)
#define tr(it,container) for(typeof(container.begin()) it = container.begin(); \
                             it != container.end(); ++it)
#define mp(a,b) make_pair((a),(b))
#define eq ==

typedef long long ll;
typedef complex<double> point;

// up right down left
const int dy[] = {-1,0,1,0};
const int dx[] = {0,1,0,-1};

const double EPS = 1e-9;
const int days[]     = {31,28,31,30,31,30,31,31,30,31,30,31};
const int daysleap[] = {31,29,31,30,31,30,31,31,30,31,30,31};

int main(){
    int k;
    cin >> k;
    for(int kk = 0;kk<k;kk++){
        string a,b;
        cin >> a;
        cin >> b;

        string ret;

        int n = max(a.size()+1,b.size()+1);
        {
            stringstream ss;
            ss << setfill('0') << setw(n) << a;
            ss >> a;
        }
        {
            stringstream ss;
            ss << setfill('0') << setw(n) << b;
            ss >> b;
        }
        string c;
        c.resize(n);
        int up = 0;
        for(int i=a.size()-1;i>=0;i--){
            int now = a[i]-'0' + b[i]-'0' + up;
            if(now >= 10){
                up = now / 10;
                now -= 10;
            }else{
                up = 0;
            }
            c[i] = now+'0';
        }
        string s;
        for(int i=0;i<n;i++){
            if(c[i] != '0'){
                for(int j=i;j<n;j++){
                    s += c[j];
                }
                goto end;

            }
        }
    end:
        cout << s << endl;
    }
    return 0;
}