# n! done recursively
# ECS 10, May 26, 2009
# Matt Bishop

# function to compute n!
# parameter: n, a non-negative integer
# returns: n!
def fact(n):
    # base case: n = 0
    if n == 0:
        return 1
    # recursion here
    else:
        return n * fact(n-1)

# wrapper for n!; check inputs
# parameter: n, a non-negative integer
# returns: n! on success, -1 on error
# calls: fact(n): computes n!
def fullfact(n):
    # quick check for numeric type
    try:
        n += 0
    except TypeError:
        return -1
    # check to be sure it's non-negative
    if n < 0:
        return -1
    # now check that it is an int
    if n != int(n):
        return -1
    # it's okay
    return fact(n)