#include <iostream>

using namespace std;
typedef long long ll;
ll n, ans;
string s;

int main()
{
    cin >> n >> s;
    for (int i = 0; i < n; i++)
        ans ^= ll(s[i]);
    if (!ans){
        cout << "Ok";
        return 0;
    }
    cout << (char)(ans);
    return 0;
}