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 main()
    N = parse(Int, readline())
    c = readline()

    r = 0
    w = 0
    for i in 1:N
        if c[i] == 'R'
            r += 1
        else
            w += 1
        end
    end

    if r == N
        println(0)
        return
    end

    if w == N
        println(N)
        return
    end

    mw = 0
    for i in 1:r
        if c[i] == 'W'
            mw += 1
        end
    end

    mr = 0
    for i in r:N
        if c[i] == 'R'
            mr += 1
        end
    end

    println(min(mr, mw))

    #if mr == mw
    #    println(mr)
    #elseif mw > mr
    #    println(mr + (mw - mr))
    #else
    #    println(mw + (mr - mw))
    #end
end

main()