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 PHP (7.4.4) (judge verdict: Time Limit Exceeded)

<?php
$N = (int)trim(fgets(STDIN));
$S = trim(fgets(STDIN));
//list($N,$D) = array_map('intval',explode(" ",trim(fgets(STDIN))));
//$A = array_map('intval',explode(" ",trim(fgets(STDIN))));


//全部置き換えるか、
//最初のRの左側にRを集めた後に右側に残ったやつを置き換えるのがよさそう
$len = strlen($S);
$ans=0;
for($i=0;$i<$len;$i++) {
  if ($S[$i]=="W") {
    //Rが残っていたら置き換える
    $rpos = strrpos($S,"R");
    if ($rpos!==false  && $rpos > $i) {
      $S[$rpos]="W";
      $ans++;
    }
  }
  else {
    $S[$i]="-";
  }
}
printf("%d\n",$ans);