# fist value is a placeholder, number of days per month
month_days = [0, 31, 28, 31 ,30, 31, 30 ,31, 31, 30, 31, 30, 31]

def is_leap(year):
    
    return year % 4 == 0 and (year % 100 != 0 or year % 400 == 0)

def days_in_month(year, month):
   
    if not 1 <= month <= 12:
        return 'Invalid month'

    if month == 2 and is_leap(year):
        return 29

    return month_days[month]


# print(is_leap(2020)) # True
# print(is_leap(2017)) # False

print(days_in_month(2018, 2)) # 28