#include<iostream>
#include<string>
using namespace std;

char getChar(string& str)
{
    for (int i = 0; i < str.size(); ++i)
    {
        int Index1 = str.find(str[i]);
        int Index2 = str.rfind(str[i]);
        if (Index1 == Index2)
        {
            return str[i];
        }
    }
    return -1;
}

int main()
{
    string str;
    while (cin >> str)
    {
        char c = getChar(str);
        if (c == -1)
        {
            cout << -1 << endl;
        }
        else
            cout << c << endl;
    }
    return 0;
}