process.stdin.resume();
process.stdin.setEncoding('utf8');

var lines = [];
var reader = require('readline').createInterface({
  input: process.stdin,
  output: process.stdout
});
reader.on('line', (line) => {
  lines.push(line);
});
reader.on('close', () => {
    const s = lines[0].split("").sort();
    let flg = false;
    s.reduce((a,v)=>{
        if(a===v) flg=true;
        return v;
    },"")
    console.log(!flg?"yes":"no");
    
});
