function Main (S) {
  S = S.split("\n")[0].split("");
  var oche = [];
  var ans = "yes";
  for (var i of S) {
    if (oche.some(inp => inp == i)) ans = "no";
    oche.push(i);
  }
  console.log(ans);
}

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