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)

#define _USE_MATH_DEFINES

#include<bits/stdc++.h>

typedef long long ll;

using namespace std;

int main() {
	int n;
	string s;
	cin >> n >> s;

	string ideal;
	int red_cnt = 0;
	int white_cnt = 0;
	for (int i=0; i<n; i++) {
		if (s[i] == 'R') {
			red_cnt++;
		} else {
			white_cnt++;
		}
	}
	int diff = 0;
	for (int i=0; i<red_cnt;i++) {
		if (s[i] != 'R') {
			diff++;
		}
	}
	for (int i=red_cnt; i<(int)s.size(); i++) {
		if (s[i] != 'W') {
			diff++;
		}
	}
	cout << diff/2 << endl;
}