# Computes n!, or the product of the integers 1 through n.
# pre : n >= 0
def factorial(n):
    if n < 0:
        raise ValueError("n must be greater or equal to 0")
    product = 1
    for i in range(2, n + 1):
        product *= i
    return product

def main():
    for i in range(0, 11):
        print(str(i) + "! =", factorial(i))
    print(factorial(-1))   # produce an error

main()