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 <vector>
#include<string>
#include<sstream>
#define CAPACITY 9


namespace jisaku{//stoi?????¨?????????????????????????????????????????§??£?¨???????

	int stoi(std::string str){
		int ret;
		std::stringstream ss;
		ss << str;
		ss >> ret;
		return ret;
	}

     std::string to_string(int i){
		 std::string text;
		std::stringstream textline;
		textline<< i;
		textline >> text;
		return text;
	}

}




class OverDigit{

public:

	int num_a[CAPACITY];
	int num_b[CAPACITY];
	OverDigit(std::string str_a, std::string str_b);
	void ChangeDigit(std::string str, int num[]);
	void Add();
};

OverDigit::OverDigit(std::string str_a, std::string str_b){

	ChangeDigit(str_a, num_a);
	ChangeDigit(str_b, num_b);

}

void OverDigit::ChangeDigit(std::string str, int num[]){

	std::vector <int> num_str(CAPACITY, 0);
	int loc = CAPACITY, k = 8;
	int  str_rest,str_Max;

	str_rest = str.length() % CAPACITY;
	str_Max = str.length() / 9;

	for (int j = str.length() - CAPACITY; str_Max > 0; j = j - CAPACITY){
		num_str[k] = jisaku::stoi(str.substr(j, CAPACITY));
		loc = k;
		k--;
		str_Max--;

	}
	//?????????????????????????????????????????\??????

	if (str_rest != 0)num_str[loc - 1] = jisaku::stoi(str.substr(0, str_rest));

	for (int j = 0; j < CAPACITY; j++){
		num[j] = num_str[j];
	}
}
//80????????§????¶???????
void OverDigit::Add(){

	int num_ans[CAPACITY] = {};
	int fat, overflow = 0, count = 0;
	std::string text,t;


	for (int j = CAPACITY - 1; j >0; j--){
		if (num_a[j] + num_b[j] <= 999999999){
			num_ans[j] = num_a[j] + num_b[j];
		}
		else if (num_a[j] + num_b[j] > 999999999){
			fat = (num_a[j] + num_b[j]) - 1000000000;
			num_ans[j] = fat;
			num_a[j - 1]++;
		}

	}

	if (num_a[0] + num_b[0] > 99999999){
		overflow = 1;
	}
	else{
		num_ans[0] = num_a[0] + num_b[0];


	}

	if (overflow == 1){
		std::cout << "overflow" << std::endl;
	}
	else {
		for (int j = 0; j < CAPACITY; j++){
			if (num_ans[j] != 0){
				count++;
				t = jisaku::to_string(num_ans[j]);
				if (t.length() != 9){
					for (int l = 0; l < (9 - t.length()); l++){
						text += "0";
					}
				}
				text += t;


				std::cout << text<<std::endl;
			}
			else if (num_ans[j] == 0 && count >= 1){
					text+="000000000";
					std::cout << text<<std::endl;
			}

		}std::cout <<text<< std::endl;
	}

};


int main(){


	//vector???????????¨??????80???????¨????
	int N;
	std::string str_a, str_b;

	std::cin >> N;
	std::cin.ignore();

	for (int i = 0; i < N; i++){
		getline(std::cin, str_a);
		getline(std::cin, str_b);
		if (str_a == "0" && str_b == "0"){std::cout << '0' << std::endl;
		}
		else if (str_a.length() >= 81 || str_b.length() >= 81){
		
			std::cout << "overflow" << std::endl;
		}
		else{

			OverDigit OverDigit_a(str_a, str_b);//???????????§??????
			OverDigit_a.Add();
		}
	}
	return 0;
}

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

using namespace std;

int main() {
    int N;
    cin >> N;
    while(N--) {
        string s1, s2;
        cin >> s1 >> s2;

        if(s1.size() > 80 || s2.size() > 80) {
            cout << "overflow" << endl;
            continue;
        }
        char buf[81] = {0};
        int p1 = s1.size()-1, p2 = s2.size()-1, pb = 79;
        int carry = 0;
        while(p2 >= 0) buf[pb--] = s2[p2--];
        pb = 79;
        while(p1 >= 0) {
            int bd = buf[pb];
            if(bd > 0) bd -= '0';
            int d = s1[p1--]-'0' + bd + carry;
            carry = d / 10;
            d %= 10;
            buf[pb--] = d+'0';
        }
        if(pb == -1 && carry > 0) cout << "overflow" << endl;
        else {
            if(carry > 0) buf[pb--] = carry+'0';
            cout << (char*)(buf+(pb+1)) << endl;
        }
    }
    return 0;
}

C++: #include <stdio.h>
#include <string.h>

#define BUF_SIZE 256

int main(void) {
	char buf[BUF_SIZE];
	char num1[100];
	char num2[100];
	char sum[100];
	char str[100];
	int n;
	int i, j, k;
	
	fgets(buf, sizeof(buf), stdin);
	sscanf(buf, "%d", &n);
	
	for (i=0; i<n; i++) {
		for (j=0; j<100; j++) {
			num1[j] = num2[j] = sum[j] = 0;
		}
		
		// first integer
		fgets(buf, sizeof(buf), stdin);
		sscanf(buf, "%s", &str);
		if (strlen(str) > 80) {
			printf("overflow\n");
			continue;
		}
		
		for (j=0; j<strlen(str); j++)  ;
		for (k=0; j>0; k++) {
			j--;
			num1[k] = str[j] - '0';
		}
		
		// second integer
		fgets(buf, sizeof(buf), stdin);
		sscanf(buf, "%s", &str);
		if (strlen(str) > 80) {
			printf("overflow\n");
			continue;
		}
		
		for (j=0; j<strlen(str); j++)  ;
		for (k=0; j>0; k++) {
			j--;
			num2[k] = str[j] - '0';
		}
		
		// sum
		for (j=0; j<80; j++) {
			sum[j] += num1[j]+num2[j];
			if (sum[j] >= 10) {
				sum[j+1]++;
				sum[j] = sum[j]%10;
			}
		}
		
		if (sum[80] != 0) {
			printf("overflow\n");
		}
		else {
			for (j=79; sum[j]==0; j--)  ;
			for (; j>=0; j--) {
				printf("%d", sum[j]);
			}
			printf("\n");
		}
	}
	
	return 0;
}

C++: #include <iostream>
#include <algorithm>
using namespace std;
#define repeat(i,n) for (int i = 0; i < (n); ++i)
#define MAX_DIGIT (80)
int d[MAX_DIGIT+1];
int main() {
    int N; cin >> N;
    repeat (test,N) {
        fill(d,d+MAX_DIGIT+1,0);
        repeat (i,2) {
            string s; cin >> s;
            if (80 < s.length()) {
                cout << "overflow" << endl;
                if (i == 0) cin >> s;
                goto next;
            }
            repeat (j,(int)s.length()) {
                d[j] += s[s.length()-1-j] - '0';
            }
        }
        repeat (i,MAX_DIGIT) {
            d[i+1] += d[i] / 10;
            d[i] %= 10;
        }
        if (d[MAX_DIGIT] != 0) {
            cout << "overflow" << endl;
        } else {
            int i = MAX_DIGIT;
            while (d[i] == 0) -- i;
            for (; 0 <= i; --i) cout << d[i];
            cout << endl;
        }
next: ;
    }
    return 0;
}

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

int main(){

	int n, s, t, cnt1, cnt2;
	string str1, str2, str3;

	cin >> n;
	
	for(int i=0; i<n; i++){
		cin >> str1 >> str2;

		str3 = "";
		cnt1 = str1.length() - 1;
		cnt2 = str2.length() - 1;
		if(cnt1 > 80 || cnt2 > 80){
			cout << "overflow" << endl;
			continue;
		}
		t = 0;
		while(cnt1 >= 0 || cnt2 >= 0){
			s = t;
			if(cnt1 >= 0) s += str1[cnt1] - '0';
			if(cnt2 >= 0) s += str2[cnt2] - '0';
			str3 += '0' + s % 10;
			t = (s - s % 10) / 10;
			cnt1--;
			cnt2--;
		}
		if(t != 0) str3 += '0' + t;
		
		if(str3.length() > 80){
			cout << "overflow" << endl;
			continue;
		}
		
		for(int j=str3.length() - 1; j>=0; j--){
			cout << str3[j];
		}
		cout << endl;
	}

  return 0;
}