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<bits/stdc++.h>
#define ll long long
#define pb push_back
 using namespace std;
  int main()
  {
      int n;
       cin>>n;
       vector<char>v;
       for(int i=0; i<n; i++)
       {
            char c;
             cin>>c;
             v.pb(c);
       }
       int i,j; 
       i=0;
        j=v.size()-1;
         int count=0;
        while(i<j)
        {
           if(v[i]=='R' && v[j]=='R')
             i++, j--;
          
            if(v[i]=='W' && v[j]=='R')
            {
              i++,j--;
                 count++;
                 
            }
          else if(v[i]=='R' && v[j]=='W')
             j--;
            
        }
        
        cout<<count<<endl;
       
       
  }