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 <stdio.h>
int main()
{
	int i,W = 0,R = 0,N,m,ans = 0;
	char WR[200000];
	scanf("%d", &N);
	WR[N + 1] = '\0';
	for (i = 0; i <= N; i++) {
		scanf("%c", &WR[i]);
	}
	for (i = 0; i <= N; i++) {
		if (WR[i] == 'R') {
			R++;
		}
		if (WR[i] == 'W') {
			W++;
		}
	}
	if (W < R) {
		printf("%d", W);
	}
	if (R < W) {
		printf("%d", R);
	}
	if (W == R) {
		for (i = 0; i <= N / 2; i++) {
			if (WR[i] == 'W') {
				for (m = N / 2; m <= N; m++) {
					if (WR[m] == 'R') {
						WR[i] = 'R';
						WR[m] = 'W';
						ans++;
					}
				}
			}
		}
	}
	printf("%d",ans);
}