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: Time Limit Exceeded)

#include <iostream>
#include <cmath>
#include <vector>
using namespace std;

int main(void){
  int N;
  cin>>N;
  vector<int> c(N,0);
  
  for(int i=0;i<N;i++){
   char c_i;
    cin>>c_i;
    if(c_i=='W')c[i]=1;
  }
  bool flag =true;
  int ans=0;
  do{
      flag=true;
      for(int i=0;i<N-1;i++){
        if(c[i]-c[i+1]>0){flag=false;ans++;break;}
      }
  	  if(!flag){
       for(int i=0;i<N;i++){
       	if(c[i]==1){
         for(int j=N-1;j>i;j--){
         	if(c[j]==0){
               c[i]=0;
               c[j]=1;
               break;
            }
         }
          break;
        }
       }
      }
  }while(!flag);
  cout<<ans<<endl;
  return 0;
}