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: Time Limit Exceeded)

function solve()
    N = parse(Int, readline())
    c = [x for x in readline()]
    R = sum((x->x=='R').(c))
    cnt = 0

    for i = 1:N
        #println("debug : ", i , ", ", c)
        if(R == 0)break end
        if(c[i] == 'R')
            R -= 1
            continue
        end
        
        R -= 1
        cnt += 1
        c[i] = 'R'
        for j = N:-1:1
            if(c[j] == 'R')
                c[j] = 'W'
                break
            end
        end
    end

    println(cnt)
end

solve()