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 Ruby (2.7.1) (judge verdict: Time Limit Exceeded)

input = readlines()

n = input.shift.chomp.to_i
list = input.shift.chomp.split('').map{|x| x.to_s}

w_list = []
w_min = nil
r_list = []
r_max = nil

list.each_with_index do |l, i|
  w_list.push(i) if l == 'W'
  r_list.push(i) if l == 'R'
end

if w_list.length == 0 || r_list.length == 0
  puts 0
  exit
end


w_min = w_list.min
r_max = r_list.max

if w_min > r_max
  puts 0
  exit
end

cnt = 0
while(w_min < r_max)
  cnt += 1
  w_target = w_list.min
  r_target = r_list.max
  w_list.delete(w_target)
  r_list.delete(r_target)
  w_list.push(r_target)
  r_list.push(w_target)

  w_min = w_list.min
  r_max = r_list.max
end

puts cnt