void main(){
  import std.stdio, std.string, std.conv, std.algorithm;

  auto s=readln.chomp.to!(char[]);

  bool[char] map;
  foreach(c; s){
    if(c in map){
      writeln("no");
      return;
    }else{
      map[c]=true;
    }
  }
  writeln("yes");

}

void rd(T...)(ref T x){
  import std.stdio, std.string, std.conv;
  auto l=readln.split;
  assert(l.length==x.length);
  foreach(i, ref e; x) e=l[i].to!(typeof(e));
}
