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: Wrong Answer)

#include<bits/stdc++.h>
using namespace std;
void lomuto(char s[],long &l,long &r){
	while(s[l]!='W')
		l++;
	while(s[r]!='R')
		r--;
	if(l>r)return;
	swap(s[l],s[r]);l++;r--;
}
bool isSafe(char s[],long n){
	for(long i=0;i<n-1;i++)
		if(s[i]=='W'&& s[i+1]=='R')
			return false;
	return true;
}
long solve(char s[],long n){
	if(isSafe(s,n))
		return 0;
	long l=0,r=n-1;
	long c=0;
	while(r>l){
		lomuto(s,l,r);
		c++;
	}
	return c;

}

int main(){
	long n;char s[200000]="";
	cin>>n;
	cin>>s;
	cout<<solve(s,n);
	return 0;
}