
"use strict"

function main(input) {
  const abc = 'abcdefghijklmnopqrstuvwxyz' // 0-25
  const alfabets = abc.split('')
  input = input.split('').sort()

  let cnt = -1;
  for (let i = 0; i < input.length; i++) {

    let index = -1;
    for (let j = 0; j < alfabets.length; j++) {
      if (input[i] == alfabets[j]) {
        index = j
      }
    }
    if (index != cnt && index != cnt + 1) {
      console.log(alfabets[cnt + 1])
      return
    } else {
      cnt = index
    }
  }
  if (cnt == 24) {

    console.log('z')

  } else {
    console.log('None')
  }
}
main(require("fs").readFileSync("/dev/stdin", "utf8"))