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>
#include <stdlib.h>

void	swap(char *i, char *j)
{
	char tmp;
	tmp = *i;
	*i = *j;
	*j = tmp;
}

int main()
{
	int N;
	scanf("%d", &N);
	char *c;
	c = malloc(N * sizeof(char));
	scanf("%s", c);
	int ans = 0;
	int i = 0;
	int j = N - 1;
	// 最初に出てきたWを探す
	for (; i < j; i++)
	{
		if (c[i] == 'W')
		{
			for (; i < j; j--)
			{
				if (c[j] == 'R')
				{
					swap(&c[i], &c[j]);
					ans++;
					break;
				}
			}
		}
	}
	printf("%d\n", ans);

	return (0);
}