Problem:
Score : 400 points

Problem Statement

An altar enshrines N stones arranged in a row from left to right. The color of the i-th stone from the left (1 \leq i \leq N) is given to you as a character c_i; R stands for red and W stands for white.

You can do the following two kinds of operations any number of times in any order:

Choose two stones (not necessarily adjacent) and swap them.

Choose one stone and change its color (from red to white and vice versa).

According to a fortune-teller, a white stone placed to the immediate left of a red stone will bring a disaster. At least how many operations are needed to reach a situation without such a white stone?

Constraints

2 \leq N \leq 200000

c_i is R or W.

Input

Input is given from Standard Input in the following format:

N
c_{1}c_{2}...c_{N}

Output

Print an integer representing the minimum number of operations needed.

Sample Input 1

4
WWRR

Sample Output 1

2

For example, the two operations below will achieve the objective.

Swap the 1-st and 3-rd stones from the left, resulting in RWWR.

Change the color of the 4-th stone from the left, resulting in RWWW.

Sample Input 2

2
RR

Sample Output 2

0

It can be the case that no operation is needed.

Sample Input 3

8
WRWWRWRR

Sample Output 3

3

Solution in OCaml (4.10.0) (judge verdict: Accepted)

let () =
  let red = ref 0 in
  let rec charlist_of_string str n i =
    if i = n then []
    else if str.[i] = 'R' then (red := !red + 1; 'R' :: charlist_of_string str n (i+1))
    else 'W' :: charlist_of_string str n (i+1)  in
  let rec count lst target n i =
    if i = n then 0
    else match lst with
      | [] -> 0
      | h :: t -> if h = target then 1 + count t target n (i+1) else count t target n (i+1) in
  Scanf.scanf "%d\n%s\n" @@ fun n s ->
  let lst = charlist_of_string s (String.length s) 0 in
  Printf.printf "%d\n" (count lst 'W' (!red) 0)