from datetime import datetime, timedelta

def calculate_dates(last_period_date, period_length, cycle_length):
    # Calculate the next period date
    next_period_date = last_period_date + timedelta(days=cycle_length)

    # Period end date
    period_end_date = last_period_date + timedelta(days=period_length)

    # The fertile window starts 3 to 5 days after the period ends and lasts for about 6 days.
    # We'll calculate the start and end dates of this ovulation period.
    ovulation_start_date = period_end_date + timedelta(days=3)  # Start of the fertile window
    ovulation_end_date = period_end_date + timedelta(days=5) + timedelta(days=6)  # End of the fertile window

    return next_period_date, ovulation_start_date, ovulation_end_date

def get_date_input(prompt):
    while True:
        date_str = input(prompt)
        try:
            # Try to convert the input string to a datetime object
            return datetime.strptime(date_str, "%Y-%m-%d")
        except ValueError:
            # If input is not a valid date, prompt the user to enter a new value
            print("Invalid date format. Please enter the date in YYYY-MM-DD format.")

def get_integer_input(prompt):
    while True:
        try:
            # Try to convert the input to an integer
            return int(input(prompt))
        except ValueError:
            # If input is not an integer, prompt the user to enter a new value
            print("Invalid input. Please enter a valid number.")

# Get user inputs
print("Please enter the following information:")
last_period_date = get_date_input("Last period start date (YYYY-MM-DD): ")
period_length = get_integer_input("How many days does the period last?: ")
cycle_length = get_integer_input("What is the cycle length in days (from the start of one period to the start of the next)?: ")

# Calculate the next period and ovulation dates
next_period, ovulation_start, ovulation_end = calculate_dates(last_period_date, period_length, cycle_length)

# Print the results
print("\nNext Period Start Date:", next_period.strftime("%Y-%m-%d"))
print("Estimated Ovulation Period: From", ovulation_start.strftime("%Y-%m-%d"), "to", ovulation_end.strftime("%Y-%m-%d"))