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

function Main(args){
    var input = args.split("\n");
    var S = input[0];
    var diffFlag = true;

    for(var i = 0; i<S.length; i++){
        for(var j=i+1; j<=S.length; j++){
            if(S[i] === S[j]){
                diffFlag = false;
                break;
            }
        }
    }

    console.log( diffFlag ? 'yes' : 'no' );
}