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 n = read_int ()
let c = read_line()

let rec r i s =
  if i = n then s
  else if c.[i] = 'R' then
    r (i + 1) (s + 1)
  else
    r (i + 1) s

let rec f i w r =
  if w == r then r
  else if c.[i] = 'W' then
    f (i + 1) (w + 1) r
  else
    f (i + 1) w (r - 1)

let () =
  let ans = f 0 0 (r 0 0) in
  Printf.printf "%d\n" ans