#include <iostream> #include <string> using namespace std; int main() { string str; char result; bool flag = false; while(cin>>str) { int length = str.length(); for(int i=length -1 ; i>=0; i--) { int pos = str.find(str[i]); if( pos == i) { if(str.find(str[i], i+1) == str.npos) { result = str[i]; flag = true; } } } if(flag) { cout << result <<endl; flag = false; } else { cout<<"-1"<<endl; } } return 0; }