    function main( str ){
      var result = "yes";
      for( var i=0; i<str.length; i++ ){
        var cnt = 0;
        for( var k=0; k<str.length; k++ ){
          if( str[i] == str[k] ) cnt++;
        }
        if( cnt > 1 ) result = "no";
      }
      console.log( result );
    }
     
    main(require("fs").readFileSync("/dev/stdin", "utf8"));