public static long repeatedString(String s, long n) {
    // Write your code here
        /*
            A easy approach to be seen is:
            - Seperate problem into the number of 'a' in full string and in substring
            + Count 'a' in full string and multiply it with a number of duplication.
            + Count 'a' in substring which has (n % l) letters with l = s.length()
            
            Solution:
            1. Calculate 'a' in substring, add it to variable "total"
            2. Calculate 'a' in full string, then multiply with the number of duplication and add to "total"
        */
        long count = 0, total = 0;
        int k = (int)(n % s.length());
        for(int i = 0; i < k; i++){
            if(s.charAt(i) == 'a') count++;
        }
        
        total += count; //number of 'a' in the substring
        for(int i = k; i < s.length(); i++){
            if(s.charAt(i) == 'a') count++;
        }
        
        //count now is the number of 'a' in the full string
        long duplication = n / s.length();
        total += count * duplication;
        
        return total;
    }