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# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.IO;
using System.Collections;
using System.Collections.Generic;
using System.Linq;

class CompetitiveProgrammingProgram
{
    static long Gcd(long a, long b)
    {
        long[] v = new long[] { a, b };
        while (v[1] != 0) { v = new long[] { v[1], v[0] % v[1] }; }
        return v[0];
    }
    static long Lcm(long a, long b)
    {
        return a / Gcd(a, b) * b;
    }
    class Node<T>
    {
        private int m_height;
        private Node<T> m_parent;
        public T Item { get; private set; }
        public Node(T item)
        {
            m_height = 0;
            m_parent = this;
            Item = item;
        }
        public Node<T> Find()
        {
            if (m_parent == this) { return this; }

            Node<T> parent = m_parent.Find();
            m_parent = parent;
            return parent;
        }
        public static void Unite(Node<T> a, Node<T> b)
        {
            Node<T> p = a.Find();
            Node<T> q = b.Find();
            if (p.m_height < q.m_height)
            {
                p.m_parent = q;
                q.m_height = Math.Max(p.m_height + 1, q.m_height);
            }
            else
            {
                q.m_parent = p;
                p.m_height = Math.Max(q.m_height + 1, p.m_height);
            }
        }
    }

    static void Main(string[] args)
    {
        int N = int.Parse(Console.ReadLine());
        string c = Console.ReadLine();
        int rcnt = 0;
        int wcnt = 0;
        for (int i = 0; i < N; i++)
        {
            if (c[i] == 'R')
                rcnt++;
            if (c[i] == 'W')
                wcnt++;
        }
        int ans = 0;
        for (int i = rcnt; i < N; i++)
        {
            if (c[i] == 'R')
                ans++;
        }
        Console.WriteLine(ans.ToString());
    }
}