Write a program to calculate the factorial of N.

Sure, here is a simple program in Python that calculates the factorial of a number N:

def factorial(N):
    if N == 0:
        return 1
    else:
        return N * factorial(N-1)

N = int(input("Enter a number: "))
print("The factorial of", N, "is", factorial(N))

Here's how it works:

1. The function factorial(N) is defined. This function will calculate the factorial of the number N.

2. Inside the function, there is a condition that checks if N is equal to 0. If it is, the function returns 1. This is because the factorial of 0 is 1.

3. If N is not 0, the function returns N multiplied by the factorial of N-1. This is a recursive call, meaning the function calls itself with a different argument.

4. Outside the function, the program asks the user to enter a number and assigns it to N.

5. Finally, the program prints the factorial of N by calling the factorial(N) function.