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: Runtime Error)

#include<bits/stdc++.h>
using namespace std;
int main()
{
int N,i,numw,startR,endW,a,ans,R,W;
  numw=0;
  char str[N];
  cin >> N >> str;
  
  for(i=0;i<N;++i)
  {
  if(str[i]=='W') numw++;
  }
  a=0;
  while(str[a]=='R')
  {
  a++;
  }
  startR=a;
  R=N-numw-startR;
  
  
  a=N-1;
  endW=0;
  while(str[a]=='W')
  {
  a--;
  endW++;
  }
  W=numw-endW;
  
  
  if(W<R) ans=W;
  else ans=R;
  
cout << ans << endl;
return 0;
}