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: Wrong Answer)

#include <iostream>
#include <bits/stdc++.h>
#include <vector>
#include <string>
#include <string.h>

using namespace std;
#define ll long long int

int main(){
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);

	ll n,r=0,w=0,rh,wh;
	cin>>n;
	string s;
	cin>>s;
	for (int i = 0; i < n; ++i)
	{
		if(s[i]=='R') r++;
		else w++;
		if(i==(n-1)/2){
			wh=w;
			rh=r;
		}
	}
	ll ans=min(r,w);
	if(r==0 || w==0)cout<<"0"<<endl;
	else{
		ans=min(ans,r-rh);
		cout<<ans<<endl;
	}

	return 0;
}