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: Runtime Error)

function AlterAltar()
    data = readlines()
    reg = r"^R+W*$"
    occursin(reg, data[2]) && return println(0)
    C = [d=='R' ? true : false for d in data[2]]
    R = sum(C)
    W = length(C) - R
    sand = match(r"^R+W*R+", data[2]).match
    W_first = length(match(r"W+", sand).match)
    right = data[2][length(sand)+1:end]
    R_left = sum([r=='R' ? 1 : 0 for r in right])
    ans = max(W_first, R_left)
    return println(ans)
end
AlterAltar()