def is_prime(num):
    if num <= 1:
        return False
    for i in range(2, int(num ** 0.5) + 1):
        if num % i == 0:
            return False
    return True

while True:
    try:
        num = int(input("Enter a positive integer: "))
        if num <= 0:
            raise ValueError()
        break
    except ValueError:
        print("Invalid input. Please enter a positive integer.")

if is_prime(num):
    print(f"{num} is a prime number.")
else:
    print(f"{num} is not a prime number.")