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: Runtime Error)

#include <stdio.h>
#include <math.h>
 
int main(){
  int N;
  int count = 0;
  int i,j;
  unsigned char tmp;
  
  scanf("%d",&N);
  char G[N+1];
  scanf("%s",G);
  
  i = 0;
  j = N-1;
  do{
    while(G[i] != 'W') i++;
    while(G[j] != 'R') j--;
    if(G[i] == 'W' && G[j] == 'R'){
      tmp = G[i];
      G[i] = G[j];
      G[j] = tmp;
      count++;
    }
  }while(i<j);
  printf("%d\n",count-1);
  return 0;
}