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<math.h>
char s[200005];
void swap(char *a,char *b){
	char c=*a;
	*a=*b;
	*b=c;
}
int main(){
	int n,ans=0;
	scanf("%d",&n);
	scanf("%s",s);
	int left=0,right=n-1;
	while(left<right){
		if(s[left]=='W'&&s[right]=='R'){
			swap(&s[left],&s[right]);
			left++;
			right--;	
			ans++;
		}
		else if(s[left]!='W')left++;
		else if(s[right]!='R')right--;
	} 
	printf("%d",ans);
	return 0;
}