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<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++){

		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 = (int)log10((double)ret);
			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 = (int)log10((double)ret);
				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 = (int)log10((double)ret);
				result.push_back(num);
			}
			if (ret != 0)
				result.push_back(ret);
		}


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

	}


	return 0;
}

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

int main()
{
	string s1, s2, temp;
	int sum, flag;
	int a, b;
	vector<int> v;
	int T;
	cin >> T;
	while (T--)
	{
		cin >> s1 >> s2;
		v.clear();
		sum = 0;
		flag = 0;
		reverse(s1.begin(), s1.end());
		reverse(s2.begin(), s2.end());
		if (s1.size() < s2.size())
		{
			temp = s1;
			s1 = s2;
			s2 = temp;
		}
		for (int i = 0; i < s1.size(); ++i)
		{
			if (s1[i] == '0')a = 0;
			else if (s1[i] == '1')a = 1;
			else if (s1[i] == '2')a = 2;
			else if (s1[i] == '3')a = 3;
			else if (s1[i] == '4')a = 4;
			else if (s1[i] == '5')a = 5;
			else if (s1[i] == '6')a = 6;
			else if (s1[i] == '7')a = 7;
			else if (s1[i] == '8')a = 8;
			else if (s1[i] == '9')a = 9;
			if (i >= s2.size())b = 0;
			else
			{
				if (s2[i] == '0')b = 0;
				else if (s2[i] == '1')b = 1;
				else if (s2[i] == '2')b = 2;
				else if (s2[i] == '3')b = 3;
				else if (s2[i] == '4')b = 4;
				else if (s2[i] == '5')b = 5;
				else if (s2[i] == '6')b = 6;
				else if (s2[i] == '7')b = 7;
				else if (s2[i] == '8')b = 8;
				else if (s2[i] == '9')b = 9;
			}
			sum = a + b + flag;
			if (sum > 9)
			{
				sum = sum - 10;
				flag = 1;
			}
			else
			{
				flag = 0;
			}
			v.push_back(sum);
		}
		if (flag == 1)
			v.push_back(1);
		if (v.size() > 80)
			cout << "overflow" << endl;
		else
		{
			for (int i = v.size() - 1; i >= 0; --i)
			{
				cout << v[i];
			}
			cout << endl;
		}
	}
	return 0;
}

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

using namespace std;

int main(){
	int n, c, res;
	bool f;
	string opr[2], buf;
	cin >> n;
	for(int i = 0;i < n;++i){
		c = 0;
		f = false;
		for(int j = 0;j < 2;++j){
			cin >> buf;
			if(buf.length() > 80) break;
			for(int k = 0;k < buf.length();++k) buf[k] = buf[k] - '0';
			opr[j].clear();
			for(int k = 0;k < 80 - buf.length();++k) opr[j] += (char)(0);
			opr[j] += buf;
		}
		if(buf.length() > 80){
			cout << "overflow" << endl;
			continue;
		}
		for(int j = opr[0].length() - 1;0 <= j;--j){
			res = opr[0][j] + opr[1][j] + c;
			opr[0][j] = res % 10;
			c = res / 10;
		}
		if(!c){
			for(int j = 0;j < opr[0].length();++j){
				if(!f) f = opr[0][j] != 0;
				if(f) cout << (char)(opr[0][j] + '0');
			}
		}
		else cout << "overflow";
		cout << endl;
	}
}

C++: #include<bits/stdc++.h>
#define rep(i,n) for(int i=0;i<n;++i)
#define FOR(i,a,b) for(int i=a;i<=b;++i)
#define LL long long
using namespace std;
static const LL INF = 1LL<<61LL;
typedef pair<int,int> PII;

string a,b;
string ans;
int M;
char t[]={'0','1','2','3','4','5','6','7','8','9'};

void add(int n,int m,int c,string s){
    int k;
    if(n-1<0&&m-1<0){
        if(c!=0)s+=t[c];
        for(int i=s.size()-1;i>=0;--i){
            ans+=s[i];
        }
        return ;
    }
    if(n-1>=0&&m-1>=0)k=a[n-1]-'0'+b[m-1]-'0'+c;
    else if(n-1>=0)k=a[n-1]-'0'+c;
    else if(m-1>=0)k=b[m-1]-'0'+c;
    if(k>=10){
        k-=10;
        c=1;
    }
    add(n-1,m-1,c,s+t[k]);
}

/*bool MOD1(string num){
    bool res=false;
    LL k=num.size();
    LL mod;
    int i=-1;
    LL n=0;
    while(k){
        n=(n*10)+num[i+1]-'0';
        mod=n%M;
        n=mod;
        k--;
        i++;
    }
    if(mod==0)return true;
    return false;
}
bool MOD2(string num){
    bool res=false;
    LL k=num.size();
    LL mod;
    int i=-1;
    LL n=0;
    while(k){
        n=(n*10)+num[i+1]-'0';
        mod=n%5;
        n=mod;
        k--;
        i++;
    }
    if(mod==0)return true;
    return false;
}
bool MOD3(string num){
    bool res=false;
    LL k=num.size();
    LL mod;
    int i=-1;
    LL n=0;
    while(k){
        n=(n*10)+num[i+1]-'0';
        mod=n%7;
        n=mod;
        k--;
        i++;
    }
    if(mod==0)return true;
    return false;
}
bool MOD4(string num){
    bool res=false;
    LL k=num.size();
    LL mod;
    int i=-1;
    LL n=0;
    while(k){
        n=(n*10)+num[i+1]-'0';
        mod=n%11;
        n=mod;
        k--;
        i++;
    }
    if(mod==0)return true;
    return false;
}*/
int main(){
    int n;
    cin>>n;
    rep(i,n){
    cin>>a>>b;
    //cin>>M;
        add(a.size(),b.size(),0,"");
        //if(!MOD1(ans)&&!MOD2(ans)&&!MOD3(ans)&&!MOD4(ans))cout<<ans<<endl;
        //else cout<<"king is angry"<<endl;
        if(ans.size()<=80&&a.size()<=80&&b.size()<=80)cout<<ans<<endl;
        else cout<<"overflow"<<endl;
        ans.clear();
    }
}