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 <stdio.h>

#define N 80

// Longint <- String
int init(int a[], char s[]);
// a <- b + c
int add(int a[], int b[], int c[]);
// print Longint
void print(int a[]);

int main(void){
	int n;
	int a[N], b[N], sum[N];
	char s[101], t[101];
	
	scanf("%d", &n);
	for (int i=0; i<n; i++){
		scanf("%s", s);
		scanf("%s", t);
		if (init(a, s)==-1 || init(b, t)==-1){
			puts("overflow");
			continue;
		}
		if (add(sum, a, b)==-1){
			puts("overflow");
		} else {
			print(sum);
		}
	}
	return 0;
}

// Longint <- String
int init(int a[], char s[]){
	int len=0;
	
	for (int i=0; i<N; i++) a[i] = 0;
	for (char *p=s; *p!='\0'; p++) len++;
	if (len>N) return -1;
	for (int i=0; s[i]!='\0'; i++){
		a[N-len+i] = s[i] - '0';
	}
	return 0;
}
// a <- b + c
int add(int a[], int b[], int c[]){
	int cry=0;
	for (int i=N-1; i>=0; i--){
		a[i] = b[i]+c[i]+cry;
		cry = a[i]/10;
		a[i] %= 10;
	}
	return cry ? -1 : 0;
}

// print Longint
void print(int a[]){
	int i;
	for (i=0; i<N && a[i]==0; i++)
	;
	if (i==N){
		puts("0");
		return;
	} else {
		for (; i<N; i++){
			printf("%d", a[i]);
		}
		puts("");
	}
}

C++: #include<stdio.h>
#include<iostream>
#include<math.h>
#include<algorithm>
#include<string>
using namespace std;

int main() {
	int n;
	cin >> n;
	for(int k = 0; k < n; k++) {
		string str1, str2;
		cin >> str1 >> str2;
		int a = str1.size(), b = str2.size();
		if(a > 80 || b > 80) {
			cout << "overflow" <<endl;
		} else if(a >= b) {
			for(int i = 0; i < b; i++) {
				str1[a - b + i] += str2[i] - '0';
				for(int i = a - 1; i >= 0; i--) {
					if(str1[i] > '9') {                 //?????????//
						if(i != 0) {
							str1[i] = str1[i] - ':' + '0';
							str1[i - 1] = str1[i - 1] + 1;
						}
					}
				}
			}
			if(a == 80 && str1[0] > '9') {
				cout << "overflow" <<endl;
			} else {
				for (int j = 0; j < a; j++) {
					if(str1[j] > '9') {
						printf("1%c",str1[j] - ':' + '0');
					} else cout<< str1[j];
				}
			}
		} else if(b > a) {
			for(int i = 0; i < a; i++) {
				str2[b - a + i] += str1[i] - '0';
				for(int i = b - 1; i >= 0; i--) {
					if(str2[i] > '9') {                    //?????????//
						if(i != 0) {
							str2[i] = str2[i] - ':' + '0';
							str2[i - 1] = str2[i - 1] + 1;
						}
					}
				}
			}
			if(b == 80 && str2[0] > '9') {
					cout << "overflow" <<endl;
			} else {
				for (int j = 0; j < b; j++) { 
					if(str2[j] > '9') {
						printf("1%c",str2[j] - ':' + '0');
					} else cout << str2[j];
				}
			}
		}	
		cout <<endl;
	}
	return 0;
}