Go: package main

import (
        "fmt"
        "bufio"
        "os"
)

func main() {
        var n int
        var a, b string
        sc := bufio.NewScanner(os.Stdin)
        fmt.Scanf("%d", &n)
        for i := 0; i < n; i++ {
                sc.Scan()
                a = sc.Text()
                sc.Scan()
                b = sc.Text()

                if len(a) > len(b) {
                        s := add(a, b)
                        if s[len(a)] > 0 {
                                if len(a) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(a)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                } else {
                        s := add(b, a)
                        if s[len(b)] > 0 {
                                if len(b) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(b)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                }
        }
}

// expects ln > lm
func add(n string, m string) (s [100]int) {
        ln := len(n)
        lm := len(m)
        var sum [100]int

        for i := 0; i < ln; i++ {
                var s, t int
                s = int(n[ln-1-i]) - int('0')

                if lm - 1 - i >= 0 {
                        t = int(m[lm-1-i]) - int('0')
                } else {
                        t = 0
                }

                sum[i+1] = (s + t + sum[i]) / 10
                sum[i] = (s + t + sum[i]) % 10
        }

        return sum
}

C++: # include <iostream>
# include <algorithm>
# include <vector>
# include <string>
# include <set>
# include <map>
# include <cmath>
# include <iomanip>
# include <functional>
# include <utility>
# include <stack>
# include <queue>
# include <list>
using namespace std;
using LL = long long;
constexpr int MOD = 1000000000 + 7;
//?¶????????????????????????????????????§??????????????????????¢????????????????
constexpr int INF = 2000000000;
const double PI = acos(-1);

string sum_string(string a, string b) {
	int al = a.size(), bl = b.size();
	reverse(a.begin(), a.end());
	reverse(b.begin(), b.end());
	string ret;
	int tmp = 0;
	for (int i = 0; i < al || i < bl || tmp; i++) {
		tmp += (i < al ? a[i] - '0' : 0) + (i < bl ? b[i] - '0' : 0);
		ret.push_back('0' + tmp % 10);
		tmp /= 10;
	}
	reverse(ret.begin(), ret.end());
	return ret;
}

int main() {
	int n;
	cin>>n;
	string a, b;
	for (int i = 0; i < n; i++) {
		cin >> a >> b;
		string ans = sum_string(a, b);
		cout << (ans.size() > 80 ? string("overflow") : ans) << endl;
	}
}

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

using namespace std;

#define MAX_INT_LENGTH 80

//large integer upto 80 digits
struct largeInt{
    int vals[MAX_INT_LENGTH]; // MAX_INT_LENGTH digits
};

void toLargeInt(string str, largeInt &lv){
    int start = MAX_INT_LENGTH-str.length();
    for (int i = 0; i < MAX_INT_LENGTH; i++){
        if (i >= start)
            lv.vals[i] = (str[i-start] - '0');
        else
            lv.vals[i] = 0;
    }
}

largeInt addLargeInt(const largeInt &lv1, const largeInt &lv2){
    largeInt sum;
    int carry = 0;
    for (int i = MAX_INT_LENGTH-1; i >= 0; i--){
        int sub_sum = lv1.vals[i] + lv2.vals[i] + carry;
        sum.vals[i] = sub_sum % 10;
        carry = sub_sum / 10;
    }
    return sum;
}

string toString(const largeInt &v){
    string str(MAX_INT_LENGTH,'0');
    for (int i = 0; i < MAX_INT_LENGTH; i++)
        str[i] = v.vals[i] + '0';
    return str;
}

int main()
{
    //ifstream cin("input.txt");
    
    // input data
    int n;
    cin >> n;
    for (int i = 0; i < n; i++){
        string v1,v2;
        cin >> v1 >> v2;
        largeInt lv1, lv2;
        toLargeInt(v1,lv1);
        toLargeInt(v2,lv2);
        largeInt sum = addLargeInt(lv1,lv2);
        cout << toString(sum) << endl;
    }
   return 0;
}

C++: #include <stdio.h>
#include <string.h>
#include <stdlib.h>
int main(void)
{
	char num1[82],num2[82];
	int a1[82],a2[82],ans[82];
	int i,j,n,k,l1,l2,kuriagari=0;
	scanf("%d",&n);
	for(i=0;i<n;i++)
	{
		for(j=0;j<82;j++)
		{
			a1[j] = 0;
			a2[j] = 0;
			ans[j] = 0;
		}
		scanf("%s",num1);
		scanf("%s",num2);
		l1 = strlen(num1);
		l2 = strlen(num2);
		for(j=0;j<l1;j++)
		{
			a1[j] = num1[l1-1-j] - '0';
		}
		for(j=0;j<l2;j++)
		{
			a2[j] = num2[l2-1-j] - '0';
		}
		for(j=0;j<82;j++)
		{
			ans[j] = (a1[j] + a2[j] + kuriagari) % 10;
			if((int)((a1[j] + a2[j] + kuriagari)/10) == 1)
			{
				kuriagari = 1;
			}
			else
			{
				kuriagari = 0;
			}
		}
		if(ans[80] != 0)
		{
			printf("overflow\n");
		}
		else
		{
			for(j=81;j>=0;j--)
			{
				if(ans[j] != 0)
				{
					k = j;
					break;
				}
			}
			for(j=k;j>=0;j--)
			{
				printf("%d",ans[j]);
			}
			printf("\n");
		}
	}
	return 0;
}