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<string.h>
#include<stdlib.h>
#include<math.h>
#define PI 3.141592653589793
#define MOD 1000000007
//#define DEBUG

//qsort用、昇順
int compare_int(const void *a, const void *b)
{
	return *(int*)a-*(int*)b;
}
//qsort(array, 10, sizeof(int), compare_int)

int main(){
	int n;
	char color[200100];
	scanf("%d", &n);
	for(int i=0; i<n; i++) scanf(" %c", &color[i]);

	int left=0, right=n-1;
	int count = 0;
	
	while((color[left] == 'R') && (left<n)) left++;
	while((color[right] == 'W') && (right>=0)) right--;
	while(left<right){
		color[left] = 'R';
		color[right] = 'W';
		count++;
		while((color[left] == 'R') && (left<n)) left++;
		while((color[right] == 'W') && (right>=0)) right--;
	}
	
	printf("%d\n", count);

	
	return 0;
}