#include <iostream>
#include <string>//данная библиотека содержит методы и переменные для работы со строками
using namespace std;
int main() {
int n;
cin >> n;
if (n % 2 == 0) cout << "Ok";
else {
int x[200];
for ( int i = 65; i <= 122; i++)
x[i] = 0;
char c;
while ((c = cin.get()) != EOF)
x[c]++;
for (int i = 'A'; i <= 'z'; i++)
if (x[i] % 2 != 0)
c = i;
cout << c;
}
return 0;
}
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