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# (Mono-mcs 6.8.0.105) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;
using System.IO;
class d{
    public static void Main(){
        int n = int.Parse(Console.ReadLine());
        char[] str = Console.ReadLine().ToCharArray();
        int left=0, right=n-1;
        int ans = 0;
        search(ref left, ref right, n, str);
        while(left < right){
            Swap(ref str[left], ref str[right]);
            ans++;
            search(ref left, ref right, n, str);
        }
        Console.WriteLine(ans);
    }
    public static void search(ref int left, ref int right, int n, char[] arr){
        for(;left < n; left++){
            if(arr[left] == 'W'){
                break;
            }
        }
        for(; right >= 0; right--){
            if(arr[right] == 'R'){
                break;
            }
        }
    }
    public static void Swap<T>(ref T a, ref T b){
        var t = a;
        a = b;
        b = t;
    }
}