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: Wrong Answer)

#include <iostream>
#include <stdio.h>
#include <vector>
#include <algorithm>
#include <cmath>
#include <string>

using namespace std;

int main() {
  string stone;
  int N,cn=0;
  cin >> N;
  cin.ignore();
  getline(cin,stone);
  int left=0,right=N-1;
  while(left < right) {
    while(left < N && stone[left]!='W') left++;
    while(right > 0 && stone[right]!='R') right--;
    //cout << left << right << endl;
    if (left > right) break;
    stone[left] = 'R';
    stone[right] = 'W';
    //left++;right--;
    cn++;
  }
  cout << cn << endl;
  return 0;
}