#include<iostream>
#include<cstdio>
#include<string>
using namespace std;
int main()
{
    string A;
    while(getline(cin,A))
    {
        for(int i=0;i<A.size();i++){
            if(A.find(A[i])==A.rfind(A[i])){
                cout<<A[i]<<endl;
                return 0;
            }
        }
    }
    cout<<"no"<<endl;
    return 0;
}