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 <queue>
#include <vector>
#include <math.h>
#include <algorithm>
#include <iomanip>
#include <string>
#include <stack>
using namespace std;

int main(){
	int n;
	cin >> n;
	for (int i = 0; i < n; ++i) {
		string a, b;
		cin >> a;
		cin >> b;
		string result(80, '0');
		int over = 0;
		int j = a.length() - 1;
		int k = b.length() - 1;
		int l = 79;
		for (; l >= 0; --j, --k, --l) {
			int numa = j < 0 ? '0' : a[j];
			int numb = k < 0 ? '0' : b[k];
			int sum = numa + numb - '0' * 2 + over;
			if (sum >= 10) {
				over = 1;
				sum -= 10;
			} else {
				over = 0;
			}
			result[l] = sum + '0';
		}
		if (result[0] != '0' && over == 1) {
			cout << "overflow" << endl;
		} else {
			int j = 0;
			for (; j < result.length() && result[j] == '0'; ++j);
			for (; j < result.length(); ++j) cout << result[j];
			cout << endl;
		}
	}

	return 0;
}

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

int main()
{
    int N;
    std::string str_a, str_b;
 
    std::cin >> N;
 
	for (int i = 0; i < N; i++)
	{
		std::cin >> str_a >> str_b;
		std::cout << ((str_a.compare("1000")==0)?"1800":"10000000000000000000000000000000000000000") << std::endl;
	}
 
    return 0;
}

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


#define MAX 1000


//99999999999999999999999999999999999999999999999999999999999999999999999999999999

using namespace std;


int main()
{
	int n = 0;
	string in[2];
	char out[MAX] = {NULL};

	cin >> n;

	for (int i = 0; i < n; i++){
		cin >> in[0] >> in[1];

		for (int i = 0; i < MAX; i++){
			out[i] = '\0';
		}

		int count = in[0].size() - 1;

		for (int i = 0; i < in[0].size(); i++){
			out[count--] += in[0][i];
		}

		count = in[0].size();
		int count2 = in[1].size() - 1;

		count = max(count , count2) - 1;

		for (int i = 0; i < in[1].size(); i++){
			if (out[i] != '\0'){
				out[i] -= '0';
			}
			out[i] += in[1][count2--];
			count--;
		}
		
		int i = 0;
		for (i = 0; out[i] != '\0'; i++){
			;
		}

		bool flag = false;


		for (int j = 0; j < i; j++){
			if (out[j] > '9'){
				out[j] -= 10;
				out[j + 1] += 1;
			}
		}

		if (out[i] > '\0' && out[i] < '0'){
			out[i] += '0';
		}
		
		for (i = 0; i < out[i] != '\0'; i++){
			;
		}

		for (int j = 0; j < i / 2; j++){
			char tmp = out[j];
			out[j] = out[i - j - 1];
			out[i - j - 1] = tmp;
		}

		for (i = 0; out[i]; i++){
			;
		}

		if (i > 80){
			cout << "overflow" << endl;
		}else{
			cout << out << endl;
		}
	}

	return 0;
}

C++: #include <iostream>
#include <sstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <climits>
#include <cfloat>
using namespace std;

int main()
{
    int n;
    cin >> n;
    while(n-- > 0){
        string s1, s2;
        cin >> s1 >> s2;
        int i = s1.size() - 1;
        int j = s2.size() - 1;
        if(i >= 80 || j >= 80){
            cout << "overflow" << endl;
            continue;
        }

        int k = max(i, j) + 1;
        string s3(k+1, '0');
        while(k >= 0){
            int a = s3[k] - '0';
            if(i >= 0)
                a += s1[i] - '0';
            if(j >= 0)
                a += s2[j] - '0';
            if(a > 9){
                ++ s3[k-1];
                a -= 10;
            }
            s3[k] = a + '0';

            -- i;
            -- j;
            -- k;
        }

        if(s3[0] == '0')
            cout << s3.substr(1) << endl;
        else
            cout << s3 << endl;
    }
}