#include <bits/stdc++.h>
using namespace std;

int main() {
  string s;
  cin>>s;
  int ansa=0;
  int ansb=0;
  int ansc=0;
  int ansd=0;
  int anse=0;
  int ansf=0;
  int ansg=0;
  int ansh=0;
  int ansi=0;
  int ansj=0;
  int ansk=0;
  int ansl=0;
  int ansm=0;
  int ansn=0;
  int anso=0;
  int ansp=0;
  int ansq=0;
  int ansr=0;
  int anss=0;
  int anst=0;
  int ansu=0;
  int ansv=0;
  int answ=0;
  int ansx=0;
  int ansy=0;
  int ansz=0;
  for(int i=0;i<s.size();i++) {
    if(s.at(i)=='a') {
      ansa++;
    }
    else if(s.at(i)=='b') {
      ansb++;
    }
    else if(s.at(i)=='c') {
      ansc++;
    }
    else if(s.at(i)=='d') {
      ansd++;
    }
    else if(s.at(i)=='e') {
      anse++;
    }
    else if(s.at(i)=='f') {
      ansf++;
    }
    else if(s.at(i)=='g') {
      ansg++;
    }
    else if(s.at(i)=='h') {
      ansh++;
    }
    else if(s.at(i)=='i') {
      ansi++;
    }
    else if(s.at(i)=='j') {
      ansj++;
    }
    else if(s.at(i)=='k') {
      ansk++;
    } 
    else if(s.at(i)=='l') {
      ansl++;
    }
    else if(s.at(i)=='m') {
      ansm++;
    }
    else if(s.at(i)=='n') {
      ansn++;
    }
    else if(s.at(i)=='o') {
      anso++;
    } 
    else if(s.at(i)=='p') {
      ansp++;
    }
    else if(s.at(i)=='q') {
      ansq++;
    }
    else if(s.at(i)=='r') {
      ansr++;
    }
    else if(s.at(i)=='s') {
      anss++;
    }
    else if(s.at(i)=='t') {
      anst++;
    }
    else if(s.at(i)=='u') {
      ansu++;
    }
    else if(s.at(i)=='v') {
      ansv++;
    }
    else if(s.at(i)=='w') {
      answ++;
    }
    else if(s.at(i)=='x') {
      ansx++;
    }
    else if(s.at(i)=='y') {
      ansy++;
    }
    else if(s.at(i)=='z') {
      ansz++;
    }
  }
  if(ansa==0) {
    cout<<"a"<<endl;
    goto end;
  } 
  else if(ansb==0) {
    cout<<"b"<<endl;
    goto end;
  } 
  else if(ansc==0) {
    cout<<"c"<<endl;
    goto end;
  }
  else if(ansd==0) {
    cout<<"d"<<endl;
    goto end;
  }
  else if(anse==0) {
    cout<<"e"<<endl;
    goto end;
  }
  else if(ansf==0) {
    cout<<"f"<<endl;
    goto end;
  }
  else if(ansg==0) {
    cout<<"g"<<endl;
    goto end;
  }
  else if(ansh==0) {
    cout<<"h"<<endl;
    goto end;
  }
  else if(ansi==0) {
    cout<<"i"<<endl;
    goto end;
  }
  else if(ansj==0) {
    cout<<"j"<<endl;
    goto end;
  }
  else if(ansk==0) {
    cout<<"k"<<endl;
    goto end;
  }
  else if(ansl==0) {
    cout<<"l"<<endl;
    goto end;
  }
  else if(ansm==0) {
    cout<<"m"<<endl;
    goto end;
  }
  else if(ansn==0) {
    cout<<"n"<<endl;
    goto end;
  }
  else if(anso==0) {
    cout<<"o"<<endl;
    goto end;
  }
  else if(ansp==0) {
    cout<<"p"<<endl;
    goto end;
  }
  else if(ansq==0) {
    cout<<"q"<<endl;
    goto end;
  }
  else if(ansr==0) {
    cout<<"r"<<endl;
    goto end;
  }
  else if(anss==0) {
    cout<<"s"<<endl;
    goto end;
  }
  else if(anst==0) {
    cout<<"t"<<endl;
    goto end;
  }
  else if(ansu==0) {
    cout<<"u"<<endl;
    goto end;
  }
  else if(ansv==0) {
    cout<<"v"<<endl;
    goto end;
  }
  else if(answ==0) {
    cout<<"w"<<endl;
    goto end;
  }
  else if(ansy==0) {
    cout<<"y"<<endl;
    goto end;
  }
  else if(ansz==0) {
    cout<<"z"<<endl;
    goto end;
  }
  else {
    cout<<"None"<<endl;
    goto end;
  }
  end: 
  return 0;
}

Measured memory_bytes: 512