(*
2^(15) = 32768 and the sum of its digits is 3 + 2 + 7 + 6 + 8 = 26.

What is the sum of the digits of the number 2^(1000)?
*)

open Big_int;;

let b0 = big_int_of_int 0;;
let b10 = big_int_of_int 10;;
let bn = power_int_positive_int 2 1000;;

let rec sumup bn s = 
  if(eq_big_int bn b0) then s else
  sumup (div_big_int bn b10) ( (int_of_big_int (mod_big_int bn b10))+s )
;;

Printf.printf "%s\n" (string_of_big_int bn);;
Printf.printf "%d\n" (sumup bn 0);;