def is_year_leap(year):
    if year % 4 == 0:
        return True
    else:
        return False

def days_in_month(year, month):

    days = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31]

    if is_year_leap(year) == True:
        days[1] = 29

    return days[month - 1]

def day_of_year(year, month, day):
    if month < 1 or month > 12:
        return None
    elif day < 1 or day > 31:
        return None
    elif day > 29 and is_year_leap(year) == True and month == 2:
        return None
    elif day > 28 and is_year_leap(year) == False and month == 2:
        return None

    day_ = day

    for i in range(1,month):
        mo = days_in_month(year,i)
        day_ += mo

    return day_

print(day_of_year(2001,2,29))