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 static System.Console;
using static System.Math;
using System.Numerics;
using System.Collections.Generic;
using System.Linq;


namespace AtCoder
{
    public class Program
    {
        public static void Main(string[] args)
        {
            int num = myRead.readInt();
            var s = myRead.reatStr();
            var rn = 0;
            var r = 0;
            var w = 0;
            for (int i = 0; i < num; i++)
            {
                rn += (s[i] == 'R') ? 1 : 0;
            }
            for (int i = 0; i < num; i++)
            {
                if (i < rn)
                    w += (s[i] == 'W') ? 1 : 0;
                else
                    r += (s[i] == 'R') ? 1 : 0;
            }
            WriteLine(2 * w - r);
            Out.Flush();
        }
    }
    public static class myRead
    {
        public static int readInt() { return int.Parse(ReadLine()); }
        public static long readLong() { return long.Parse(ReadLine()); }
        public static string reatStr() { return ReadLine(); }
        public static int[] readIntArray() { return ReadLine().Split(' ').Select(int.Parse).ToArray(); }
        public static long[] readLongArray() { return ReadLine().Split(' ').Select(long.Parse).ToArray(); }
        public static string[] readStrArray() { return ReadLine().Split(' '); }
    }


    public static class MyMath
    {
        public static long Factorical(long n)
        { return n <= 1 ? 1L : n * Factorical(n - 1); }
        public static long Permutation(long n, long m)
        { return m == 0 ? 1L : (m == n) ? Permutation(n, m - 1) : n * Permutation(n - 1, m - 1); }
        public static long Combination(long n, long m)
        { return n - m > m ? Permutation(n, m) / Factorical(m) : Permutation(n, n - m) / Factorical(n - m); }
        public static long Gcd(long n, long m)
        { return n % m == 0 ? m : Gcd(m, n % m); }
        public static long Lcm(long n, long m)
        { return n * m / Gcd(n, m); }
        public static long Pow(long n, long m)
        { return (m == 1 || m == 0) ? 1L : n * Pow(n, m - 1); }
    }


}