-- n! means n × (n − 1) × ... × 3 × 2 × 1
--
-- For example, 10! = 10 × 9 × ... × 3 × 2 × 1 = 3628800,
-- and the sum of the digits in the number 10! is 3 + 6 + 2 + 8 + 8 + 0 + 0 = 27.
--
-- Find the sum of the digits in the number 100!

import ProjectEuler (digitSum)

factorial :: (Integral a) => a -> a
factorial 0 = 1
factorial n = n * factorial (n - 1)

main = do
    let result = digitSum $ factorial 100
    putStrLn $ "Project Euler, Problem 20\n"
        ++ "Answer: " ++ show result