class Solution {
    public int maximum69Number (int num) {
        int first6=-1;
        int number=num;
        
        for(int i=1; num>0; i++){
            if(num%10==6){
                first6=i;
            }
            num/=10;
        }
        
        return number+3*(int)Math.pow(10, first6-1);
    }
}