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.to_i
c = gets.chomp.chars
p = [c.count('W'), c.count('R')]
v = []
v[0] = (c[0] == 'W' ? 1 : 0)
n.pred.times do |i|
  v[i + 1] = v[i] + (c[i + 1] == 'W' ? 1 : 0)
end
ans = p.min
cnt = 0
mm = 0
md = p[0]
-1.downto(-n) do |i|
  if c[i] == 'R'
    mm += 1
    md = p[0] + i
    ans = mm + md if ans > mm + md
  else
    md = p[0] + i
    ans = mm + md if ans > mm + md
  end
  break if p[0] == -i
end
puts ans