/** Returns the number of leap years between year1 and year2, inclusive. 
 * Precondition: 0 <= year1 <= year2
 */
public static int numberOfLeapYears(int year1, int year2)
{
    int leapYears = 0;
    
    for(int y = year1; y<= year2; y++)
        if(isLeapYear(y))
            leapYears++;
            
    return leapYears;
    
    
    /* to be implemented in part (a) */
}