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 Fortran (GNU Fortran 9.2.1) (judge verdict: Wrong Answer)

program alter_altar
    character(200001) :: c = '0'
    integer n
    integer :: r(200001) = 0
    integer :: w(200001) = 0
    integer :: i = 1
    integer :: k = 1
    integer :: j = 0
    read *, n
    read *, c
    do
        if(i == 1)then
            w(i) = index(c,'W')
            r(k) = index(c,'R',back=.true.)
            if(w(i) < r(k))then
                c(w(i):w(i)) = 'R'
                c(r(k):r(k)) = 'W'
                j = j + 1
            end if
            if(w(i) > r(k) .or. w(i) == 0 .or. r(i) == 0) exit
       else
            w(i) = index(c(w(i+1):),'W')
            r(k) = index(c(:r(k-1)),'R',back=.true.)
            !if(w(i) > r(k))exit
            c(w(i):w(i)) = 'R'
            c(r(k):r(k)) = 'W'
            j = j + 1
        end if
    end do
    print *, j
end program