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)

def max(a, b); a < b ? b : a; end
def min(a, b); a < b ? a : b; end

class Problem
  attr_accessor *(?a..?z).to_a.map(&:to_sym)

  def initialize
    @n = gets.to_s.to_i
    @s = gets.chomp
  end

  def solve
    lo = 0
    hi = n - 1
    ans = 0
    while lo < hi
      if s[lo] == "R" && s[hi] == "W"
        lo += 1
        hi -= 1
      elsif s[lo] == "W" && s[hi] == "R"
        ans += 1
        lo += 1
        hi -= 1
      elsif s[lo] == "R" && s[hi] == "R"
        lo += 1
      else
        hi -= 1
      end
    end
    ans
  end

  def show(ans)
    puts ans
  end
end

Problem.new.instance_eval do
  show(solve)
end