function Main(input){
    input = input.split('');
    var counts = {};
    input.forEach(function(elm) {
        var key = elm;
        if(counts[key]) {
            counts[key] += 1;
        } else {
            counts[key] = 1;
        }
    });

    var result = 0;
    for(var count in counts) {
        if(counts[count] > 1) {
            result += 1;
        }
        
    }
    if(result === 0) {
        console.log('yes');
    } else {
        console.log('no');
    }

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