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: Accepted)

<?php
fscanf(STDIN, "%s", $N);
fscanf(STDIN, "%s", $X);

$count = 0;
while (true) {
    $wPos = strpos($X, 'W');
    $rPos = strrpos($X, 'R');
    if ($wPos === false || $rPos === false || $wPos >= $rPos) {
        break;
    }

    // 切り取る
    $tmp = $X;
    $tmp = substr($tmp, $wPos + 1);
    $tmp = substr($tmp, 0, -(strlen($X) - $rPos));
    $X = $tmp;
    $count++;
}
echo $count;