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 <iostream>
#include<bits/stdc++.h>
#define ll long long
using namespace std;

int main() {
ll n,res=0,i,a=0,b=0,c=0,d=0;
cin>>n;
string s;
cin>>s;
for(i=0;i<n;i++)
{
	if(s[i]=='W')
	a++;
	else
	b++;
	
}
for(i=0;i<b;i++)
{
	if(s[i]=='R')
	c++;
}
for(;i<n;i++)
{
	if(s[i]=='W')
	d++;
}

cout<<min(n-a,min(n-b,min(b-c,a-d)))<<endl;

}