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.*;
public class Main{
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		while(sc.hasNext()){
			int N = sc.nextInt();
			String str = sc.next();
			char[] chars =str.toCharArray();
			int p1 = 0, p2 = N-1;
			int op = 0;
			while(p1 < p2){
				while(p1 < p2 && chars[p1] =='R'){
					p1++;
				}
				while(p2 > p1 && chars[p2] =='W'){
					p2--;
				}
				if(p1 < p2){
					op++;
					p1++;
					p2--;
				}
			}
			System.out.println(op);
		}
	}
}