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 ABC174D
    implicit none
    integer(8)::N,i,Wc(0:200000),Rc(0:200000),ans=2000000
    character(200000)::C
    read*,N,C
    Wc(0)=0
    Rc(0)=0
    do i=1,N
        if(C(i:i)=="R")then
            Rc(i)=Rc(i-1)+1
            Wc(i)=Wc(i-1)
        else
            Rc(i)=Rc(i-1)
            Wc(i)=Wc(i-1)+1
        end if
    end do

    do i=0,N
        ans=min(ans,max(Wc(i)-Wc(0),Rc(N)-Rc(i+1)))
    end do
    print'(i0)',ans
end program ABC174D