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: Runtime Error)

using System;
using System.Collections.Generic;
using System.Windows;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace Atcoder
{
    public class Program
    {
        private static long mod = 1000000007;
        private static long max = 999999999999;

        static void Main(string[] args)
        {
            var n = Read().Int();
            var l = new string[n];
            var c = Read();
            var rl = new List<int>();
            var wl = new List<int>();
            for (int i = 0; i < n; i++)
            {
                if (c[i].ToString() == "W")
                {
                    wl.Add(i);
                }
                else
                {
                    rl.Add(i);
                }
            }
            rl.Reverse();

            var rCount = rl.Count;
            var wCount = wl.Count;

            var ans = 0;
            foreach (var item in wl)
            {
                if (rl[0] > item)
                {
                    rl.RemoveAt(0);
                    ans++;
                }
                else
                {
                    break;
                }
            }

            ans = Math.Min(ans, rCount);
            ans = Math.Min(ans, wCount);

            WriteRead(ans);
        }

        public static string Read()
        {
            return Console.ReadLine();
        }

        public static string[] ReadSplit()
        {
            return Console.ReadLine().Split(' ');
        }

        public static void WriteRead(object str)
        {
            Console.WriteLine(str);
            Read();
        }

        public static void Write(object str)
        {
            Console.WriteLine(str);
        }

        private static bool ChangeMin<T>(ref T a, T b) where T : IComparable
        {
            if (a.CompareTo(b) > 0)
            {
                a = b;
                return true;
            }
            return false;
        }

        private static bool ChangekMax<T>(ref T a, T b) where T : IComparable
        {
            if (b.CompareTo(a) > 0)
            {
                a = b;
                return true;
            }
            return false;
        }

        /*優先度付きキュー*/
        class PriorityQueue<T> where T : IComparable<T>
        {
            public readonly T[] Array;
            public readonly bool Greater;
            public int Count { get; private set; } = 0;

            public PriorityQueue(int capacity, bool greater = true)
            {
                this.Array = new T[capacity];
                this.Greater = greater;
            }

            public void Enqueue(T item)
            {
                this.Array[this.Count] = item;
                this.Count += 1;
                this.UpHeap();
            }

            public T Dequeue()
            {
                // 先頭要素を末尾にして再構成
                this.Swap(0, this.Count - 1);
                this.Count -= 1;
                this.DownHeap();

                return this.Array[this.Count];
            }


            private void UpHeap()
            {
                var n = this.Count - 1;
                while (n != 0)
                {
                    // 親要素の座標
                    var parent = (n - 1) / 2;

                    if (this.Compare(this.Array[n], this.Array[parent]) > 0)
                    {
                        this.Swap(n, parent);
                        n = parent;
                    }
                    else
                    {
                        break;
                    }
                }
            }

            private void DownHeap()
            {
                var parent = 0;
                while (true)
                {
                    var child = 2 * parent + 1;
                    if (child > this.Count - 1) break;

                    if (child < this.Count - 1 && this.Compare(this.Array[child], this.Array[child + 1]) < 0)
                    {
                        // 左より右の子のほうが大きい値の場合、右を入れ替え対象にする
                        child += 1;
                    }

                    if (this.Compare(this.Array[parent], this.Array[child]) < 0)
                    {
                        this.Swap(parent, child);
                        parent = child;
                    }
                    else
                    {
                        break;
                    }
                }
            }

            //aとbを比較する
            private int Compare(T a, T b)
            {
                var c = a.CompareTo(b);
                if (!this.Greater)
                {
                    c = -c;
                }
                return c;
            }

            //aとbを入れ替える
            private void Swap(int a, int b)
            {
                var tmp = this.Array[a];
                this.Array[a] = this.Array[b];
                this.Array[b] = tmp;
            }
        }

        private static bool NextPermutation(int[] array)
        {
            var size = array.Length;
            var ok = false;

            //array[i]<array[i+1]を満たす最大のiを求める
            int i = size - 2;
            for (; 0 <= i; i--)
            {
                if (array[i] < array[i + 1])
                {
                    ok = true;
                    break;
                }
            }

            //全ての要素が降順の場合、NextPermutationは存在しない
            if (ok == false) return false;

            //array[i]<array[j]を満たす最大のjを求める
            int j = size - 1;
            for (; ; j--)
            {
                if (array[i] < array[j]) break;
            }

            //iとjの要素をswapする
            var tmp = array[i];
            array[i] = array[j];
            array[j] = tmp;

            //i以降の要素を反転させる
            Array.Reverse(array, i + 1, size - (i + 1));

            return true;
        }

        //要素数Nを引数として渡し、0～N-1でできた順列をすべて返す
        static List<int[]> GetAllPermutation(int N)
        {
            var res = new List<int[]>();
            var array = Enumerable.Range(0, N).ToArray();
            do
            {
                var copy = new int[N];
                array.CopyTo(copy, 0);
                res.Add(copy);
            } while (NextPermutation(array));

            return res;
        }

        //配列arrayを引数として渡し,辞書順がarray以上の順列をすべて返す
        static List<int[]> GetAllPermutation(int[] array)
        {
            var size = array.Length;
            var res = new List<int[]>();
            do
            {
                var copy = new int[size];
                array.CopyTo(copy, 0);
                res.Add(copy);
            } while (NextPermutation(array));

            return res;
        }
    }
}


public static class Extentions
{
    public static int Int(this object obj)
    {
        return int.Parse(obj.ToString());
    }

    public static long Long(this object obj)
    {
        return long.Parse(obj.ToString());
    }

    public static decimal Decimal(this object obj)
    {
        return decimal.Parse(obj.ToString());
    }

    public static double Double(this object obj)
    {
        return double.Parse(obj.ToString());
    }

    public static T2 GetValueZero<T1, T2>(this Dictionary<T1, T2> d, T1 key) where T2 : struct
    {
        T2 value;
        if (d.TryGetValue(key, out value))
        {
            return value;
        }
        return new T2();
    }
}