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
c_str = gets.chomp

r_count = c_str.each_char.count {|c| c == "R" }
w_count = n - r_count

# 全部一色に変える場合の回数
min = [r_count, w_count].min

# 文字列の間に仕切りを1つ入れて, それより左の赤白の数, 右の赤白の数を管理
left = {red: 0, white: 0}
right = {red: r_count, white: w_count}
0.upto(c_str.length-1) do |i|
  if c_str[i] == "R"
    left[:red] += 1
    right[:red] -= 1
  else
    left[:white] += 1
    right[:white] -= 1
  end
  # left[:whilte]=0, right[:red]=0 にするような操作の回数を求める
  # leftにある白とrightにある赤を入れ替える操作を限界まで行って ... (*)
  # leftに残っている白の数 もしくは rightに残っている赤の数を数えるイメージ
  # (*)の操作を行うと必ず (left[:white] == 0 || right[:red] == 0) はtrueになる
  tmp = [left[:white], right[:red]].min + (left[:white] - right[:red]).abs
  min = tmp if tmp < min
end

puts min