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 Julia (1.4.0) (judge verdict: Accepted)

# SPDX-License-Identifier: X11
# 2020-09-17
# Alter Altar (400pt)

function _main()
  N = parse(Int, readline())
  C = Vector{Char}(undef, 0)
  W, R = 0, 0

  for i ∈ 1:N
    c = read(stdin, Char)
    c == 'R' && (R += 1)
    push!(C, c)
  end

  ans = R
  for c ∈ C
    c == 'R' ? R -= 1 : W += 1
    ans = min(ans, max(W,R))
  end

  println(ans)
end

_main()