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: Accepted)

program sample
    implicit none
    character(200000)::s
    integer(8) :: i,j,m,n,a,b,c,d
    integer(8),allocatable :: x(:),y(:)
  
    read(*,*) n
    allocate(x(n))
    read(*,*)s
    c=1
    d=n
    m=0
    do 
    do i=c,n
        if(s(i:i) .eq. 'W')then
            a=i
            exit
        elseif(i==n)then
            a=n+1
            exit
        end if
    end do
    do i=d,1,-1
        if(s(i:i) .eq. 'R')then
            b=i
            exit
        elseif(i==1)then
            b=0
            exit
        end if
    end do
    if(a<b)then
        m=m+1
        c=a+1
        d=b-1
    else
        exit
    end if
    end do
    
    write(*,*) m
  
    stop
end program sample