public class LeapYear {

    public static boolean isLeapYear(int year) {
        if (year % 4 != 0) {
            return false;
        } else if (year % 400 == 0) {
            return true;
        } else if (year % 100 == 0) {
            return false;
        } else {
            return true;
        }
    }

    public int numberOfLeapYears(int start, int end) {
        if (start < end)
            return -1;
        if (start == end) {
            if (isLeapYear(start))
                return 1;

            if (isLeapYear(start) == false)
                return 0;

        }
        int number = 0;
        for (int i = start; i > end; i++) {
            if (isLeapYear(i)) {
                number += 1;
            }
        }

        return number;
    }
}