'use strict'

function main(s) {
  const str = s.split('\n')[0].split('');

  const before_len = str.length;
  const after_len = [...new Set(str)].length;

  const ans = (before_len === after_len) ? 'yes' : 'no';
  console.log(ans);
}

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