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<bits/stdc++.h>
using namespace std;
const int N=200005;
int n,ans=1e9,a[2][N];
char s[N];
int zh(int x,int l,int r)
{
	if(l>r)
		return 0;
	return a[x][r]-a[x][l-1];
}
int main()
{
	scanf("%d%s",&n,s+1);
	for(int i=1;i<=n;i++)
	{
		int x=s[i]=='W';
		a[0][i]=a[0][i-1],a[1][i]=a[1][i-1];
		a[x][i]++;
	}
	for(int i=0;i<=n;i++)
	{
		int v1=zh(1,1,i),v2=zh(0,i+1,n);
		ans=min(ans,max(v1,v2));
	}
	printf("%d\n",ans);
	return 0;
}