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;
  int R=0;
  char c[200000];
  int a;
  int i,j;
  int ans;
  int numbr;
  scanf("%d",&N);
  a=getchar(); i=0;
  while(a!=EOF){
    if(a=='R'){
      R++;
    }
    c[i++]=(char)a;
    a=getchar();
  }
  c[i]='\0';
  ans=0; j=0;
  for(i=0;i<N;i++){
    if(j>=R){
      printf("%d\n",ans);
      return 0;
    }
    else if(c[i]=='W'){
      j++;
      ans++;
    }
    else if(c[i]=='R'){
      if(R<=i){
        j++;
        ans++;
      }
      else{
        j++;
      }
    }
  }
  printf("0\n");
  return 0;
}