'use strict'

function Main(input) {
  const S = input;
  const chars = new Set();
  for (let i = 0; i < S.length; i++) {
	chars.add(S[i]);
  }

  if (!chars.has('a')) {
    console.log('a');
  } else if (!chars.has('b')) {
    console.log('b');
  } else if (!chars.has('c')) {
    console.log('c');
  } else if (!chars.has('d')) {
    console.log('d');
  } else if (!chars.has('e')) {
    console.log('e');
  } else if (!chars.has('f')) {
    console.log('f');
  } else if (!chars.has('g')) {
    console.log('g');
  } else if (!chars.has('h')) {
    console.log('h');
  } else if (!chars.has('i')) {
    console.log('i');
  } else if (!chars.has('j')) {
    console.log('j');
  } else if (!chars.has('k')) {
    console.log('k');
  } else if (!chars.has('l')) {
    console.log('l');
  } else if (!chars.has('m')) {
    console.log('m');
  } else if (!chars.has('n')) {
    console.log('n');
  } else if (!chars.has('o')) {
    console.log('o');
  } else if (!chars.has('p')) {
    console.log('p');
  } else if (!chars.has('q')) {
    console.log('q');
  } else if (!chars.has('r')) {
    console.log('r');
  } else if (!chars.has('s')) {
    console.log('s');
  } else if (!chars.has('t')) {
    console.log('t');
  } else if (!chars.has('u')) {
    console.log('u');
  } else if (!chars.has('v')) {
    console.log('v');
  } else if (!chars.has('w')) {
    console.log('w');
  } else if (!chars.has('x')) {
    console.log('x');
  } else if (!chars.has('y')) {
    console.log('y');
  } else if (!chars.has('z')) {
    console.log('z');
  } else {
    console.log('None');
  }
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));