#include <iostream>
#include <algorithm>
#include <string>
using namespace std;
int main()
{
	string s;
	cin >> s;
	for (char c : s)
	{
		if (count(s.begin(), s.end(), c) == 2)
		{
			cout << c << endl;
			break;
		}
	}
	system("pause");
	return 0;
}