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;
int n;
char c[200010];
int cnt;
int cnt2;
int main()
{
	cin>>n;
	for(int i=1;i<=n;i++)
	{
		cin>>c[i];
	}
	for(int i=n;i>=1;i--)
	{
		if(c[i]=='W')
		{
			cnt=i;
			break;
		}
	}
	if(!cnt)
	{
		cout<<0;
		return 0;
	}
	for(int i=1;i<cnt;i++)
	{
		if(c[i]=='W')
		{
			cnt2++;
		}
	}
	if(cnt==n&&cnt2==n-1)
	{
		cout<<0;
		return 0;
	}
	bool flag=0;
	for(int i=1;i<n;i++)
	{
		if(c[i]!=c[i+1])
		{
			flag=1;
		}
		else
		{
			flag=0;
			break;
		}
	}
	if(cnt2>=n-cnt)
	{
		if(flag)
		{
			cnt2--;
		}
		cout<<cnt2;
	}
	else
	{
		if(flag)
		{
			cnt2--;
		}
		cout<<cnt2+1;
	}
	return 0;
}