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 Julia (1.4.0) (judge verdict: Wrong Answer)

function F()
  N = parse(BigInt,readline())
  S = Vector{Char}(readline())
  s = [1 2 3]
  
  
  C = []
  wl = 0
  rl = 0
  
  wr = count(x->x=='W',S)
  
  rr = count(x->x=='R',S)
  if wr<=wl
    C = append!(C,wl)
  else
    C = append!(C,wr)
  end
  
  for i=1:N
    if S[i] == 'W'
      wl += 1
      wr -= 1
    else
      rl += 1
      rr -= 1
    end
    if wl<=rr
      c = rr
    else
      c = wl
    end
    C = append!(C,c)
  end
  ans = minimum(C)
  println(ans)
end

F()