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: Accepted)

#include<stdio.h>

int cmp_rw(int r, int w){
  if(r>=w){
    return r;
  }else{
    return w;
  }
}

int main(){
  int n,i,w=0,r=0,min;
  scanf("%d",&n);

  char c[n+1];
  scanf("%s",c);

  for(i=0;i<n;i++){
    if(c[i]=='R'){
      r++;
    }
  }

  min = cmp_rw(r,w);

  for(i=0;i<n;i++){
    if(c[i]=='W'){
      w++;
    }else if(c[i]=='R'){
      r--;
    }
    if(cmp_rw(r,w)<min){
      min = cmp_rw(r,w);
    }
  }

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

  return 0;
}