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 Java (OpenJDK 11.0.6) (judge verdict: Accepted)

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;
import java.util.Scanner;
import java.util.Set;
import java.util.TreeMap;
import java.util.TreeSet;

public class Main {
	
	
    public static void main(String[] args) throws IOException {
    	Scanner s = new Scanner(System.in);
		PrintWriter out = new PrintWriter(System.out);
		BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
		String[] input;
		int n = Integer.parseInt(reader.readLine());
		String c = reader.readLine();
		int ans = Integer.MAX_VALUE, r = 0, w = 0;
		for (int i = 0; i < n; i++) {
			if (c.charAt(i) == 'R') {
				r++;
			}
		}
		ans = r;
		for (int i = 0; i < n; i++) {
			if (c.charAt(i) == 'R') {
				r--;
			}
			if (c.charAt(i) == 'W') {
				w++;
			}
			ans = Math.min(ans, Math.min(r, w) + Math.abs(r - w));
		}
		out.print(ans);
		out.flush();
		out.close();
    }

}