#include<iostream> #include<string> using namespace std; int main(){ string s; int a[28]={0},i; cin>>s; for(i=0;i<s.size();++i) a[s[i]-'a']++; for(i=0;i<s.size();++i) if(a[s[i]-'a']==1) break; if(i<s.size()) cout<<s[i]; else cout<<"no"; return 0; }