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 n;
  char c[200000];
  int r,w,s,x;
  r=0;w=0;s=0;x=0;
  scanf("%d",&n);
  for(int i=0;i<n;i++){
    scanf("%c",&c[i]);
    if(c[i]=='R'){
      r++;
    }else{
      w++;
    }
  }
  for(int i=0;i<r;i++){
    if(c[i]=='w'){
      s++;
    }
  }
  for(int i=r;i<n;i++){
    if(c[i]=='R'){
      x++;
    }
  }
  if(s<=x){
    printf("%d\n",x);
  }else{
    printf("%d\n",s);
  }
  return 0;
}