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.to_i
s = gets.chomp
a = s.chars

l = nil
r = nil

nr = 0
for i in 0...n
    if a[i] == 'R'
        nr += 1
        r = i
    else
        l = i unless l
    end
end

l = nr unless l

ans = 0
for i in 0...n
    r = n-1-i

    break if l >= r || l >= nr

    if a[r] == 'R'
        # swap l <-> r
        # p "swap l:#{l} r:#{r}"
        a[r] = 'W'
        a[l] = 'R'
        l += 1
        ans += 1
        while a[l] == 'R' && l < r
            l += 1
        end
    end
end

nl = n - nr

ans = nl if ans > nl
ans = nr if ans > nr

puts ans