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

  char c[N];
  char dummy;
  scanf("%c", &dummy);
  for(int i=0;i<N;i++){
    scanf("%c", &c[i]);
  }

  int d[N];
  for(int i=0;i<N;i++){
    if(c[i] == 'W') d[i] = 1;
    else  d[i] = 0;
  }

  int count = 0;
  int flag = 0;
  for(int i=0;i<N;i++){
    if(flag == 0){
      if(d[i] == 1) flag = 1;
    }
    else{
      if(d[i] == 0) count++;
    }
  }

  for(int i=0;i<N-2;i++){
    if(d[i]==0 && d[i+1]==1 && d[i+2]==0){
      int a = 1;
      for(int j=i+3;j<N;j++){
	a *= d[j];
      }
      if(a==0) count--;
    }
  }

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

  return 0;
}