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>
#define pii pair<int,int>
#define x first
#define y second
#define mp make_pair
#define pb push_back
#define ll long long
#define sz size()
#define ft front
#define pu push
#define ty empty
#define rep(i,n) for(int i=0;i<n;i++)
using namespace std;
const int m5=100042;
int main(void)
{
	int n;
	cin>>n;
	string s;
	cin>>s;
	vector<int>r;
	vector<int>w;
	for(int i=0;i<n;i++)
	{
		if(s[i]=='W')
		{
			w.pb(i);
		}
	}
	for(int i=n-1;i>=0;i--)
	{
		if(s[i]=='R')
		{
			r.pb(i);
		}
	}	
	int ans=0;
	int m=min(r.sz,w.sz);
	for(int i=0;i<m;i++)
	{
		if(w[i]<r[i])
		{
			ans++;
		}
	}
	cout<<ans<<endl;
    return 0;
}