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 <vector>
#include <set>
#include <algorithm>
#include <string>
#include <sstream>
#include <iomanip>
 
#define INPUT_FROM_FILE (0)
#if INPUT_FROM_FILE
#include <fstream>
#endif

void str_to_vec(std::string str, std::vector<char> &v){
	v.clear();
	for(int i = str.size() - 1; i >= 0; i--){
		v.push_back(str[i] - 48);
	}
	return;
}

std::vector<char> sum(std::vector<char> &v0, std::vector<char> &v1){
	std::vector<char> ret;
	char carry = 0;

	if(v0.size() < v1.size()){
		std::swap(v0, v1);
	}
	for(unsigned int i = 0; i < v1.size() - v0.size(); i++){
		v1.push_back(0);
	}

	ret.resize(v1.size(), 0);

	for(unsigned int i = 0; i < ret.size(); i++){
		char tmp = v0[i] + v1[i] + carry;
		if(tmp >= 10){
			carry = 1;
			tmp -= 10;
		}
		else{
			carry = 0;
		}
		ret[i] = tmp;
	}
	if(carry == 1){
		ret.push_back(1);
	}

	return ret;
}

int main(int argc, char **argv){
	int n = 0;
	std::string buf;

#if INPUT_FROM_FILE
	std::ifstream ifs("test.txt");
#endif



#if INPUT_FROM_FILE
	std::getline(ifs, buf);
#else
	std::getline(std::cin, buf);
#endif
	n = std::stoi(buf);

	for(int i = 0; i < n; i++){
		std::vector<char> num0;
		std::vector<char> num1;
#if INPUT_FROM_FILE
		std::getline(ifs, buf);
#else
		std::getline(std::cin, buf);
#endif
		str_to_vec(buf, num0);
#if INPUT_FROM_FILE
		std::getline(ifs, buf);
#else
		std::getline(std::cin, buf);
#endif
		str_to_vec(buf, num1);

		if(num0.size() > 80 || num1.size() > 80){
			std::cout << "overflow" << std::endl;
		}
		else{
			std::vector<char> s = sum(num0, num1);

			if(s.size() > 80){
				std::cout << "overflow" << std::endl;
			}
			else{
				for(int i = s.size() - 1; i >= 0; i--){
					std::cout << (int)s[i];
				}
				std::cout << std::endl;
			}
		}
	}
	return 0;
}

C++: #include <iostream>
#include <string>
#include <stack>
#include <math.h>
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <vector>

using namespace std;


bool f(long long x) {
	if (log10(x) >= 80) return false;
	else return true;
}



int main()
{
	int n;
	cin >> n;
	long long a;
	long long b;
	while(cin>>a){
		cin >> b;
		if (f(a) && f(b) && f(a + b)) cout << a + b << endl;
		else cout << "overflow" << endl;
	}
	

}

C++: #include<iostream>
#include<string>
#define max(a,b) a>b?a:b
using namespace std;
int che(char a){
	char i;
	int j=0;
	for(i='0';i<='9';i++){
		if(a==i){
			return j;
		}
		j++;
	}
	return 0;
}
char cha(int a){
	char i;
	int j=0;
	for(i='0';i<='9';i++){
		if(j==a){
			return i;
		}
		j++;
	}
}
int main(){
	string a,b;
	int c=0;
	int i,j,k,m,n;
	int x,y;
	char d[200];
	cin>>n;
	for(k=0;k<n;k++){
		cin>>a>>b;
		x=a.size();
		y=b.size();
		m=max(x,y);
		for(i=1;i<=m;i++){
			j=che(a[x-i])+che(b[y-i])+c;
			if(j>=10){
				c=1;
				j-=10;
			}else{
				c=0;
			}
			d[i]=cha(j);
		}
		if(c==1){
			d[i]=cha(1);
			c=0;
			i++;
		}
		cout<<i<<endl;
		if(i>81){
			cout<<"overflow"<<endl;
			continue;
		}
		for(j=i-1;j>0;j--){
			cout<<d[j];
		}
		cout<<endl;
	}
	return 0;
}

C++: #include<cstdio>

int main(){
	int a[82] = {};
	int b[82] = {};
	int c[82] = {};
	int n;
	int k,l,m,f;
	char t;
	int of;

	scanf("%d\n", &n);


	for(int i = 0; i < n; i++){
		of = k = l = 0;
		scanf("%c", &t);
		while(t - 10){
			k++;
			a[k] = t - '0';
			scanf("%c", &t);
			if(k == 81){
				of = 1;
				scanf("%c", &t);
				while(t - 10){
					scanf("%c", &t);
				}
			}
		}
		scanf("%c", &t); 

		while(t - 10){
			l++;
			b[l] = t - '0';
			scanf("%c", &t);
			if(l == 81){
				of = 1;
				scanf("%c", &t);
				while(t - 10){
					scanf("%c", &t);
				}
			}

		}

		m = 0;
		f = 0;

		while(k - m){
			c[m] = a[k - m];
			m++;
		}
		m = 0;
		while(l - m){
			c[m] += b[l - m ] + f;
			f = 0;
			if(c[m] > 9){
				c[m] -= 10;
				f = 1;
			}
			m++;
		}

		while(f){
			c[m] += 1;
			f = 0;
			if(c[m] > 9){
				c[m] -= 10;
				f = 1;
			}
			m++;
		}

		if(k > m){
			m = k;
		}

		for(int i = 0; i < m; i++){
			if(m == 81 || of){
				printf("overflow");
				i = m;
			}else{
				printf("%d", c[m - i - 1]);
			}
		}
		printf("\n");
	}
	return 0;
}