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 C (GCC 9.2.1) (judge verdict: Accepted)

#include <stdio.h>

int main(void)
{
	int n, rn, wn, lp, rp;
	char str[200005];
	int i, j, ans;

	scanf("%d", &n);
	scanf("%s", str);

	rn = wn = 0;
	for (i = 0; i < n; i++)
	{
		if (str[i] == 'R') rn++;
		else wn++;
	}

	if (rn == 0 || wn == 0) printf("0\n");
	else if (rn == 1 || wn == 1) printf("1\n");
	else
	{
		ans = 0;
		lp = 0;
		rp = n - 1;
		while (1)
		{
			for (; ; lp++)
			{
				if (str[lp] == 'W') break;
			}
			for (; ; rp--)
			{
				if (str[rp] == 'R') break;
			}
			if (lp > rp) break;
			str[lp] = 'R';
			str[rp] = 'W';
			ans++;
		}
		printf("%d\n", ans);
	}

	return 0;
}