process.stdin.resume();
process.stdin.setEncoding('utf8');
// Here your code !
process.stdin.on('data', function (chunk) {
    var lines = chunk.toString().split(' ');
    
    var L = parseInt(lines[0].length);
    var arry =[];
    
    var A =arry.fill(L);
    var ans = 0;
    
    var str = chunk.toString().split("");
    
    for(i = 0; i<L; i++){
        if(str[i+1] == str[i]){
            ans = ans +1 ;
        
        }    
    }
    
    if(ans=0){
     console.log("yes");
       }else{
     console.log("no");
           
       }
    
    
    /*if(ans>0){
     console.log("no");
       }else{
     console.log("yes");
           
       }
    */
});
            