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 1.8.0) (judge verdict: Accepted)

import java.math.BigInteger;
import java.util.*;
import java.util.stream.Collectors;

public class Main {
  public static void main(String[] args){
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    char[] c = sc.next().toCharArray();
    int rCount = 0;
    int wCount = 0;
    for (int i = 0; i < n; i++) {
      if (c[i] == 'W') {
        wCount++;
      } else {
        rCount++;
      }
    }
    int left = 0;
    int right = n - 1;
    int count = 0;
    while (true) {
      if (c[left] == 'W') {
        while (true) {
          if (c[right] != 'R') {
            right--;
            if (left >= right) {
              break;
            }
          } else {
            count++;
            right--;
            break;
          }
        }
      }
      left++;
      if (left >= right) {
        break;
      }
    }
    System.out.println(Math.min(Math.min(wCount, rCount), count));
  }
}