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++ (GCC 9.2.1) (judge verdict: Accepted)

/**
 * @author yuzining
 * @date 2020/08/02 20:10
 */

#ifdef local
#include "D:\vscode\acm\local.hpp"
#else
#include<bits/stdc++.h>
using namespace std;
#define debug(...) do { } while(false)
void Main();
int main() {
  ios::sync_with_stdio(false), cin.tie(0), cout.tie(0);
  Main();
  return 0;
}
#endif

void Main() {
  int n; cin >> n;
  string s; cin >> s;
  int w = 0, r = 0;
  for(int i = 0; i < n; i++) {
    if(s[i] == 'W') {
      w++;
    } else {
      r++;
    }
  }
  int ans = 0;
  for(int i = 0; i < r; i++) {
    if(s[i] == 'W') {
      ans++;
    }
  }
  cout << ans << '\n';
}