Project Euler 20 - Factorial Digit Sum

def factorial(n):
    if n < 0 or n != int(n):
        print("Error using factorial function")
        return -1000
    elif n == 0:
        return 1
    else:
        fact = list()
        fact.append(1)
        for i in range(1, n+2):
            fact.append(i * fact[i-1])
        return fact[n]

def countDigits(n):
    string = str(n)
    sum = 0
    for i in range(len(string)):
        ch = string[i]
        sum += int(ch)
    return sum

# MAIN FUNCTION
n = 100
fact = factorial(n)
print(str(n) + "! = " + str(fact))
print("The sum of all the digits of " + str(n) + "! is: " + str(countDigits(fact)))