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>

#define MAX 200010

int main()
{
	int N;
	int i, j;
	int count = 0;

	char str[MAX];

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

	j = N-1;

	for (i = 0 ; i < N ; i++)
	{
		if (str[i] == 'W')
		{
			while(str[j] != 'R')
			{
				j--;
				if (j <= i)
				{
					goto out;
				}
			}
			str[i] = 'R';
			str[j] = 'W';
			count++;
		}
	}

out:
	printf("%d\n", count);

	return 0;
}