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++ (Clang 10.0.0) (judge verdict: Accepted)

#include<iostream>
using namespace std;
int main()
{
  int n, i, j, count=0;
    cin >> n;
  char temp, ch[n];
 
  for(i=0; i<n; i++)
    cin >> ch[i];
  i=0;
  j=n-1;
  while(i<j)
  {
    while(ch[i]!='W' && i<n)
      i++;
    while(ch[j]!='R' && j>=0)
      j--;
    if(i < j)
    {
      count++;
      temp = ch[i];
      ch[i] = ch[j];
      ch[j] = temp;
      i++;
      j--;
    }
  }
  cout << count;
  return(0);
}