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: Wrong Answer)

N=gets.to_i
c=gets.chomp.chars
cc = c.length
mid = cc / 2
c1 = c[0, mid]
c2 = c[mid, cc]
w1c = c1.count('W')
#r1c = mid + 1 - w1c
r2c = c2.count('R')
#w2c = mid + 1 - r2c

puts [w1c, r2c].min
=begin
i=0
p c.gsub!(/WR/, '')
while i <= 200000
  break if c.scan(/WR/).empty?
  
  i += 1
end
puts i
=end