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)

#include <bits/stdc++.h>

using namespace std;
using ll = long long;

const int N = 200005;

int n;
char str[N];
int p[N];
int s[N];

int main() {
	scanf("%d %s", &n, str + 1);

	for(int i = 1; i <= n; i++) {
		p[i] = p[i - 1];
		if(str[i] == 'W') p[i]++;
	}
	for(int i = n; i >= 1; i--) {
		s[i] = s[i + 1];
		if(str[i] == 'R') s[i]++;
	}

	int ans = min(p[n], s[1]);

	for(int i = 1; i <= n; i++) {
		ans = min(ans, max(p[i], s[i + 1]));
	}

	printf("%d\n", ans);
}