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)

line = readlines.map(&:chomp)

N = line[0].to_i
S = line[1]

w_cum = []
r_cum = []

wcum = 0
for i in 1..N do
  if S[i-1] == "W" then
    wcum += 1
  end
  w_cum[i-1] = wcum
  r_cum[i-1] = i - w_cum[i-1]
end
  
r_to_w = []
w_to_r = []
for i in 1..(N+1) do
  if i == 1 then
    r_to_w[i-1] = r_cum[N-1]
    w_to_r[i-1] = 0
  elsif i <= N then
    r_to_w[i-1] = r_cum[N-1] - r_cum[i-2]
    w_to_r[i-1] = w_cum[i-2]
  else
    r_to_w[i-1] = 0
    w_to_r[i-1] = w_cum[N-1]
  end
end
    
def max(x,y)
  if x > y then
    return x
  else
    return y
  end
end
  
num_alter = []
for i in 1..(N+1) do
  num_alter[i-1] = max(r_to_w[i-1],w_to_r[i-1])
end
  
ans = 10 ** 10
for i in 1..(N+1) do
  if num_alter[i-1] < ans then
    ans = num_alter[i-1]
  end
end
puts ans