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# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Collections;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Diagnostics;

class TEST{
	static void Main(){
		Sol mySol =new Sol();
		mySol.Solve();
	}
}

class Sol{
	public void Solve(){
		
		int[] sum = new int[N + 1];
		for(int i=0;i<N;i++){
			sum[i + 1] = sum[i] + (S[i] == 'R' ? 1 : 0);
		}
		
		int min = (int) 1e9;
		
		for(int i=0;i<=N;i++){
			int lr = sum[i];
			int rr = sum[N] - sum[i];
			int lw = i - lr;
			int rw = (N - i) - rr;
			
			int cnt = 0;
			
			// 右から持ってくる
			int swap = Math.Min(lw, rr);
			cnt += swap;
			lw -= swap;
			rr -= swap;
			lr += swap;
			rw += swap;
			
			// 反転する
			int lx = lw;
			cnt += lx;
			lw -= lx;
			lr += lx;
			
			int rx = rr;
			cnt += rx;
			rr -= rx;
			rw += rx;
			
			Debug.Assert(lr == i && lw == 0 && rw == N - i && rr == 0);
			min = Math.Min(min, cnt);
		}
		
		Console.WriteLine(min);
		
		
	}
	int N;
	String S;
	public Sol(){
		N = ri();
		S = rs();
	}

	static String rs(){return Console.ReadLine();}
	static int ri(){return int.Parse(Console.ReadLine());}
	static long rl(){return long.Parse(Console.ReadLine());}
	static double rd(){return double.Parse(Console.ReadLine());}
	static String[] rsa(char sep=' '){return Console.ReadLine().Split(sep);}
	static int[] ria(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>int.Parse(e));}
	static long[] rla(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>long.Parse(e));}
	static double[] rda(char sep=' '){return Array.ConvertAll(Console.ReadLine().Split(sep),e=>double.Parse(e));}
}