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)

#pragma GCC optimize(2)
#include<bits/stdc++.h> 
using namespace std;
#define ll long long
#define endl "\n"
const int mod=1e9+7;
const int MAX=1e6+7;
int a[MAX],b[MAX];
int main(){
    ios_base::sync_with_stdio(0);cin.tie(0),cout.tie(0);
    int n,ans=0;cin>>n;
    string s; cin>>s;
    int l=0,r=0;
	for(int i=0;i<n;i++)
	{
		if(s[i]=='W')a[i]=a[i-1]+1;
		else a[i]=a[i-1];
	}
	for(int i=n-1;i>=0;i--)
	{
		if(s[i]=='R')r++;
		if(r==a[i]){
			cout<<r<<endl;return 0;
		}
	}
	cout<<0;	
   return 0;
}