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 Lua (Lua 5.3.5) (judge verdict: Accepted)

local mmi, mma = math.min, math.max
local n = io.read("*n", "*l")
local s = io.read()
local t = {}
for i = 1, n do
  t[i] = s:sub(i, i) == "W"
end
local left, right = {}, {}
do
  local c = 0
  for i = 1, n do
    if t[i] then c = c + 1 end
    left[i] = c
    right[i] = 0
  end
  c = 0
  for i = n, 1, -1 do
    if t[i] then c = c + 1 end
    right[i] = c
  end
end
local ret = mmi(n - right[1], left[n])
for i = 1, n - 1 do
  local need_red = left[i]
  local need_white = n - i - right[i + 1]
  ret = mmi(ret, mma(need_red, need_white))
end
print(ret)