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: Accepted)

n = gets.chomp.to_i
arr = gets.chomp.split('')
left, right = 0, arr.length - 1
count = 0
while true
  head_left = -1
  tail_right = -1
  left.upto(n - 1) do |i|
    if arr[i] == 'W'
      head_left = i
      break
    end
  end
  right.downto(0) do |i|
    if arr[i] == 'R'
      tail_right = i
      break
    end
  end
  break if head_left == -1 || tail_right == -1 || head_left >= tail_right
  arr[head_left], arr[tail_right] = arr[tail_right], arr[head_left]
  left = head_left
  right = tail_right
  count += 1
end
puts count