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 <stdio.h>

int main() {
	long long int n = 0, cw = 0, cr = 0, Cr = 0, ans;
	char c[200010];
	scanf("%lld%s", &n, c);
	for (int i = 0; i < n; i++) if (c[i] == 'R')Cr++;
	if (Cr == n || Cr == 0)ans = 0;
	else {
		for (long long int i = 0; i < Cr; i++)if (c[i] == 'W') cw++;
		for (long long int i = Cr - 1; i < n; i++)if (c[i] == 'R') cr++;
		if (cw < cr)ans = cw;
		else ans = cr;
	}
	printf("%lld", ans);
	return 0;
}