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.

Examples

Input

4
WWRR


Output

2


Input

2
RR


Output

0


Input

8
WRWWRWRR


Output

3
import java.util.Arrays;
import java.util.Scanner;

public class Main {
	public static void main(String[]args) {
		try(Scanner scan = new Scanner(System.in)){

			int N = scan.nextInt();

			String S = scan.next();
			char[] c = new char[N];
			c = S.toCharArray();

			int[]R = new int[N];Arrays.fill(R, 1000000);
			int[]W = new int[N];Arrays.fill(W,1000000);

			int k = 0;
			for(int i = 0;i<N;i++) {
				if(c[i]=='W') {
					W[k] = i;k++;
				}
			}

			int j = 0;
			for(int i = N-1;i>=0;i--) {
				if(c[i]=='R') {
					R[j] = i;j++;
				}
			}

			int count = 0;
			for(int i = 0;i<N;i++) {
				//System.out.println(W[i]+" "+R[i]);
				
				if(W[i]==1000000||R[i]==1000000)break;
				
				if(W[i]<R[i])count++;
				else break;

			}


			System.out.println(count);




		}


	}





}
