Project Euler #016

2¹⁵ = 32768 and the sum of its digits is 3 + 2 + 7 + 6 + 8 = 26. What is the sum of the digits of the number 2¹⁰⁰⁰?

let rec exp n e =
    match e = 0 with
    | true  -> 1I
    | false -> n * exp n (e - 1) 

let sum n =
    n |> string |> Seq.fold (fun acc x -> acc + System.Int32.Parse(x |> string)) 0

exp 2I 1000 |> sum