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>
#include <stdlib.h>
#include <string.h>

int main(void)
{
  int n;
  scanf("%d", &n);

  char *c;
  c = (char *)malloc(sizeof(char)*n);
  scanf("%s", c);

  int i = 0;
  int j = n-1;
  int m = 0;
  while(i<j) {
    while((i < n) && ('W' != c[i])) {
      i++;
    }
    while((0 < j) && ('R' != c[j])) {
      j--;
    }

    if((i == n) || (j == 0)) {
      break;
    }

    c[i] = 'R';
    c[j] = 'W';
    i++;
    j--;
    m++;
  }

  printf("%d\n", m);

  free(c);

  return 0;
}