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++: //RE
#include<iostream>
#include<algorithm>
#include<vector>
#include<queue>
#include<deque>
#include<map>
#include<set>
#include<string>
#include<sstream>
#include<cstdio>
#include<cmath>
#include<cstring>
#include<cctype>
#include<climits>
using namespace std;
#define REP(i, j) for(int i = 0; i < j; i++)
#define FOR(i, j, k) for(int i = j; i < k; i++)
#define P pair<int, int>
const int INF = INT_MAX / 2;
const int N = 100;

vector<int> makeDx(string str){
    reverse(str.begin(), str.end());
    vector<int> ret(N, 0);
    REP(i, str.size()) ret[i] = str[i] - '0';
    return ret;
}

bool isOver80(vector<int> dx){
    FOR(i, 80, dx.size()) if(dx[i] != 0) return true;
    return false;
}

int main(){
    int n; cin >>n;
    REP(z, n){
        string str[2]; cin >>str[0] >>str[1];
        if(str[0].size() > 80 || str[1].size() > 80){
            cout <<"NA" <<endl;
            continue;
        }

        vector<int> dx[2] = {makeDx(str[0]), makeDx(str[1])};
        vector<int> ans(N, 0);

        REP(i, max(dx[0].size(), dx[1].size())){
            ans[i] += (dx[0][i] + dx[1][i]);
            ans[i + 1] += (ans[i] / 10);
            ans[i] = ans[i] % 10;
        }

        if(isOver80(ans)){
            cout <<"NA" <<endl;
            continue;
        }

        reverse(ans.begin(), ans.end());
        int s = 0;
        while(ans[s] == 0 && s < N - 1) s++;
        FOR(i, s, ans.size()) cout <<ans[i];
        cout <<endl;
    }
    return 0;
}

C++: #include<iostream>
#include<string>
#include<vector>
int main(int, char**)
{
	int number = 0;
	std::cin >> number;

	std::string buff1;
	std::string buff2;
	for (int count = 0; number > count; ++count)
	{
		std::cin >> buff1 >> buff2;

		std::vector<std::string> sum;
		sum.reserve(10);
		int carry = 0;
		for (int offset = 0; buff1.length() > offset || buff2.length() > offset; offset += 8)
		{
			auto cutoff = [](const std::string& buff, const int offset)->int
			{
				if (buff.length() <= offset)
				{
					return 0;
				}
				if (buff.length() <= offset + 8)
				{
					return std::stoi(buff.substr(0, buff.length() - offset));
				}
				return std::stoi(buff.substr(buff.length() - offset - 8, 8));
			};
			int num1 = cutoff(buff1, offset);
			int num2 = cutoff(buff2, offset);
			std::string sum_buff = std::to_string(num1 + num2 + carry);
			
			if (8 < sum_buff.length())
			{
				carry = std::stoi(sum_buff.substr(0, 1));
				sum.emplace_back(sum_buff.substr(1));
			}
			else
			{
				carry = 0;
				sum.emplace_back(sum_buff);
			}
		}
		if (10 < sum.size() || (sum.size() == 10 && sum[0].length() == 8 && 0 != carry))
		{
			std::cout << "overflow";
		}
		else
		{
			if (0 != carry)
			{
				std::cout << std::to_string(carry);
			}
			for (int index = static_cast<int>(sum.size()) - 1; 0 <= index; --index)
			{
				if (index != static_cast<int>(sum.size()) - 1 && "0" == sum[index])
				{
					std::cout << "00000000";
				}
				else
				{
					std::cout << sum[index];
				}
			}
		}
		std::cout << std::endl;
	}
}

C++: #include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <complex>
#include <stack>
#include <queue>
using namespace std;
static const double EPS = 1e-5;
typedef long long ll;
typedef pair<int,int> pint;
#define rep(i,n) for(int i=0;i<n;i++)
#define F first
#define S second
int dx[]={1,-1,0,0},dy[]={0,0,1,-1};

int N;

int f(string s1,string s2,int x){
	if(N>80){cout<<"overflow"<<endl;return 0;}
	if(s1.size()<=N&&s2.size()<=N&&x==0)return 0;
	if(s1.size()>N)x+=s1[s1.size()-N-1]-'0';
	if(s2.size()>N)x+=s2[s2.size()-N-1]-'0';
	++N;
	f(s1,s2,x/10);
	if(N<=80)cout<<x%10;
	return 0;
}

int main(){
	int n;
	cin>>n;
	rep(i,n){
		string s1,s2;
		cin>>s1>>s2;
		f(s1,s2,0);
		cout<<endl;
	}
	return 0;
}