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 <cstdio>
#include <cstdlib>
#include <string>

using namespace std;

int main()
{
	int N; cin >> N;

	for(int i=0; i<N; i++){
		int a[80], b[80], c[80];
		string s, t, m;
		cin >> s >> t;

		for(int j=0; j<80; j++){ a[j]=0; b[j]=0; c[j]=0;}
		for(int j=s.length()-1, k=0; j>=0; j--, k++){
			m=s.substr(k,1);
			if(m=="0"){ a[j]=0;}
			else if(m=="1"){ a[j]=1;}
			else if(m=="2"){ a[j]=2;}
			else if(m=="3"){ a[j]=3;}
			else if(m=="4"){ a[j]=4;}
			else if(m=="5"){ a[j]=5;}
			else if(m=="6"){ a[j]=6;}
			else if(m=="7"){ a[j]=7;}
			else if(m=="8"){ a[j]=8;}
			else{ a[j]=9;}
		}
		for(int j=t.length()-1, k=0; j>=0; j--,k++){
			m=t.substr(k,1);
			if(m=="0"){ b[j]=0;}
			else if(m=="1"){ b[j]=1;}
			else if(m=="2"){ b[j]=2;}
			else if(m=="3"){ b[j]=3;}
			else if(m=="4"){ b[j]=4;}
			else if(m=="5"){ b[j]=5;}
			else if(m=="6"){ b[j]=6;}
			else if(m=="7"){ b[j]=7;}
			else if(m=="8"){ b[j]=8;}
			else{ b[j]=9;}
		}

		for(int j=79; j>=0; j--){
			cout << a[j];
		}
		cout << endl;
		for(int j=79; j>=0; j--){
			cout << b[j];
		}
		cout << endl;
		/*		
		int d=0;
		bool flag=false;
		for(int j=0; j<80; j++){
			c[j] = (a[j]+b[j]+d)%10;
			d = (a[j]+b[j]+d)/10;
			if(j==79 && d>0){ flag=true;}
		}
		
		if(flag){cout << "overflow";}		
		else{
				flag = false;
				for(int j=79; j>=0; j--){
					if(flag){ cout << c[j];}
					else if(c[j] != 0){ cout << c[j]; flag=true;}
				}
		}
		cout << endl;*/
	}

	return 0;
}

C++: #include <iostream>
#include <string>
#include <utility>
#include <algorithm>
#include <queue>
#include <map>
#include <fstream>
#include <array>
#include <utility>
#include <functional>
#include <stack>

using namespace std;
typedef long long Int;
#define REP(i , x) for(int i = 0 ; i < x ; ++i)

typedef pair<int, int> P;

int a_a[85];
int b_a[85];
int c_a[85];

int main()
{
	int n;
	string a, b;
	while (cin >> n){
		REP(i, n){
			if (a.length() >= 80 || b.length() >= 80){
				cout << "overflow" << endl;
			}

			cin >> a >> b;
			REP(i, a.length()){
				a_a[i] = a[i] - '0';
			}
			
			REP(i, b.length()){
				b_a[i] = b[i] - '0';
			}

			REP(i, 85){
				int temp = a_a[i] + b_a[i];
				c_a[i + 1] = temp / 10;
				c_a[i] = temp % 10;
			}
			int i = 84;
			while (!c_a[i])--i;
			if (i >= 80){
				cout << "overflow" << endl;
			}
			while (i >= 0){
				cout << c_a[i];
				--i;
			}
			cout << endl;
		}
	}
	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
const int INF = 1000000000;
#define REP(i,s,n) for(int i=(int)(s);i<(int)(n);i++)
#define rep(i,n) REP(i, 0, n)
int main(){
    int test; cin >> test;
    rep(casenum, test){
        string a, b;
        cin >> a >> b;
        int n1 = a.size(), n2 = b.size();
        int n = max(n1, n2);
        if(n1 > 80 || n2 > 80){
            cout << "overflow" << endl;;
            continue;
        }
        if(n1 < n2){
            int sa = n2 - n1;
            string p = "";
            rep(i, sa) p += '0';
            a = p + a;
        }else if(n1 > n2){
            int sa = n1 - n2;
            string p = "";
            rep(i, sa) p += '0';
            b = p + b;
        }
        bool keta = false;
        string res = "";
        for(int i = n - 1; i >= 0; --i){
            int k = (a[i] - '0') + (b[i] - '0');
            if(keta) k++;
            if(k < 10){
                char temp = '0' + k;
                res = temp + res;
                keta = false;
            }else{
                char temp = '0' + (k - 10);
                res = temp + res;
                keta = true;
            }
        }
        if(keta) res = '1' + res;
        if(res.size() > 80){
            cout << "overflow" << endl;
            continue;
        }
        cout << res << endl;
    }
    return 0;
}

C++: #include "bits/stdc++.h"

#define debug(x) cout<<#x<<": "<<x<<endl
#define rep(i,n) for (int i=0;i<(n);i++)
#define FOR(i,a,b) for (int i=(a);i<=(b);i++)
#define all(a) (a).begin(),(a).end()
using namespace std;
typedef vector<int> VI;
typedef vector<vector<int>> VVI;
typedef long long ll;
ll gcd(ll a, ll b) {
	ll c;
	while (a != 0) {
		c = a; a = b%a;  b = c;
	}
	return b;
}

void solve() {
#ifdef _WIN32
	istream &cin = ifstream("input.txt");
#endif
	int n;
	cin >> n;
	rep(i, n) {
		vector<ll> v1(11), v2(11);
		string s1, s2;
		cin >> s1 >> s2;
		rep(j, 10) {
			if (s1.size() > 10 * j) v1[j] = stoi(s1.substr(10 * j, (s1.size() - 10 * j >= 10 ? 10 : s1.size() - 10 * j)));
			if (s2.size() > 10 * j) v2[j] = stoi(s2.substr(10 * j, (s2.size() - 10 * j >= 10 ? 10 : s2.size() - 10 * j)));
		}
		rep(j, 11) {
			v1[j] += v2[j];
			if (v1[j] >= 1e10) {
				v1[j] -= 1e10;
				v2[j]++;
			}
		}
		if (v1[8] || v1[9] || v1[10]) cout << "overflow" << endl;
		else {
			rep(j, 11) if (v1[10 - j] != 0) cout << v1[10 - j];
			cout << endl;
		}
	}

}




int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);


	solve();
	system("PAUSE");
	return 0;
}