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

int main(){
  // 宣言、入力
  string S;
  vector<int> vec(26);
  for(int i=0; i<26; i++){
    vec.at(i) = 0;
  }
  cin >> S;

  // 英文を一文字ずつに分けて、a=0, b=1と数字に置き換える
  // 26個の配列に0から入れていく
  // vec.at(0)から順に配列の数字を確認して0のところがあれば最初の存在しないアルファベット
  // ない場合は"None"

  for(int i=0; i<S.size(); i++){
    switch(S.at(i)){
      case 'a':
        vec[0]++;
        break;
      case 'b':
        vec[1]++;
        break;
      case 'c':
        vec[2]++;
        break;
      case 'd':
        vec[3]++;
        break;
      case 'e':
        vec[4]++;
        break;
      case 'f':
        vec[5]++;
        break;
      case 'g':
        vec[6]++;
        break;
      case 'h':
        vec[7]++;
        break;
      case 'i':
        vec[8]++;
        break;
      case 'j':
        vec[9]++;
        break;
      case 'k':
        vec[10]++;
        break;
      case 'l':
        vec[11]++;
        break;
      case 'm':
        vec[12]++;
        break;
      case 'n':
        vec[13]++;
        break;
      case 'o':
        vec[14]++;
        break;
      case 'p':
        vec[15]++;
        break;
      case 'q':
        vec[16]++;
        break;
      case 'r':
        vec[17]++;
        break;
      case 's':
        vec[18]++;
        break;
      case 't':
        vec[19]++;
        break;
      case 'u':
        vec[20]++;
        break;
      case 'v':
        vec[21]++;
        break;
      case 'w':
        vec[22]++;
        break;
      case 'x':
        vec[23]++;
        break;
      case 'y':
        vec[24]++;
        break;
      case 'z':
        vec[25]++;
        break;
    }
  }

  // for(int i=0; i<26; i++){
  //   cout << i << ":" << vec.at(i) << endl;
  // }

  int answer;
  answer = -1;
  for(int i=25; i >= 0; i--){
    if(vec.at(i)==0){
      answer = i;
    }
  }

  // cout << answer << endl;

  string letter;
  switch(answer){
    case -1:
      letter = "None";
      break;
    case 0:
      letter = "a";
      break;
    case 1:
      letter = "b";
      break;
    case 2:
      letter = "c";
      break;
    case 3:
      letter = "d";
      break;
    case 4:
      letter = "e";
      break;
    case 5:
      letter = "f";
      break;
    case 6:
      letter = "g";
      break;
    case 7:
      letter = "h";
      break;
    case 8:
      letter = "i";
      break;
    case 9:
      letter = "j";
      break;
    case 10:
      letter = "k";
      break;
    case 11:
      letter = "l";
      break;
    case 12:
      letter = "m";
      break;
    case 13:
      letter = "n";
      break;
    case 14:
      letter = "o";
      break;
    case 15:
      letter = "p";
      break;
    case 16:
      letter = "q";
      break;
    case 17:
      letter = "r";
      break;
    case 18:
      letter = "s";
      break;
    case 19:
      letter = "t";
      break;
    case 20:
      letter = "u";
      break;
    case 21:
      letter = "v";
      break;
    case 22:
      letter = "w";
      break;
    case 23:
      letter = "x";
      break;
    case 24:
      letter = "y";
      break;
    case 25:
      letter = "z";
      break;
  }

  cout << letter << endl;
}


Measured memory_bytes: 512