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
    integer i, x, y
    integer :: k = 0
    integer :: n = 0

    read *, n
    read *, c

    do i = 1, n
        x = index(c,'W')
        y = index(c,'R',back=.true.)
        if(x == 0 .or. y == 0)exit
        if(x < y)then
            c(x:x) = 'R'
            c(y:y) = 'W'
            k  = k + 1
        end if
        x = index(c,'W')
        y = index(c,'R',back=.true.)
        if(x < y)then
            c(x:x) = 'R'
            k = k + 1
        end if
        x = index(c,'W')
        y = index(c,'R', back=.true.)
        if(x > y)exit
    end do
    print *, k
end program