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: Time Limit Exceeded)

#include <bits/stdc++.h>
using namespace std;
 
#define FAST ios_base::sync_with_stdio(0); cin.tie(); cout.tie();
 
#define MAXX 10005
 
#define PI   3.14159265358979323846264338327950
 
#define F first
#define S second
 
#define ll 	 long long int
 
#define mod  1000000007



int main()
{
	FAST;
	ll n;
	cin >> n;
	string s;
	cin >> s;
	ll x = n-1;
	ll h = 0;
	for(ll i = 0; i < n -1 ; i++){
		if(s[i]== 'W' && x > i){
			for(ll j = x; j > i; j --){
			if(s[j] == 'R' || !j){
				x = j;
				break;
			}
			
		}
		if(s[i] == 'W' && s[x] == 'R' & x > i){
			swap(s[x], s[i]);
			h ++;
			x --;
		}
		}
		
	}
	cout << h;
	return 0;
}