#include<iostream>
#include<string>
using namespace std;
int vis[26];
int main(){
	string a;
	cin>>a;
	for(int i=0;i<a.size();i++){
		vis[a[i]-'a']++;		
	}
	for(int i=0;i<26;i++){
		if(vis[i]==1){
			switch(i){
				case 0:cout<<"a"<<endl;return 0;
				case 1:cout<<"b"<<endl;return 0;
				case 2:cout<<"c"<<endl;return 0;
				case 3:cout<<"d"<<endl;return 0;
				case 4:cout<<"e"<<endl;return 0;
				case 5:cout<<"f"<<endl;return 0;
				case 6:cout<<"g"<<endl;return 0;
				case 7:cout<<"h"<<endl;return 0;
				case 8:cout<<"i"<<endl;return 0;
				case 9:cout<<"j"<<endl;return 0;
				case 10:cout<<"k"<<endl;return 0;
				case 11:cout<<"l"<<endl;return 0;
				case 12:cout<<"m"<<endl;return 0;
				case 13:cout<<"n"<<endl;return 0;
				case 14:cout<<"o"<<endl;return 0;
				case 15:cout<<"p"<<endl;return 0;
				case 16:cout<<"q"<<endl;return 0;
				case 17:cout<<"r"<<endl;return 0;
				case 18:cout<<"s"<<endl;return 0;
				case 19:cout<<"t"<<endl;return 0;
				case 20:cout<<"u"<<endl;return 0;
				case 21:cout<<"v"<<endl;return 0;
				case 22:cout<<"w"<<endl;return 0;
				case 23:cout<<"x"<<endl;return 0;
				case 24:cout<<"y"<<endl;return 0;
				case 25:cout<<"z"<<endl;return 0;
			}
		}
	}
	cout<<"no"<<endl;
	return 0;
}