/**
 * @param {String} input
 */
function Main(input){
	var letterMap = {};
	while( input ){
		var letter = input.charAt(0);
		if(letterMap[letter]){
			return "no";
		}
		letterMap[letter] = true;
		input = input.slice( 1 );
	}
    return "yes"; 
}
console.log(Main(require("fs").readFileSync("/dev/stdin", "utf8")));
