Problem:
Score : 300 points

Problem Statement

There are N stones arranged in a row. Every stone is painted white or black.
A string S represents the color of the stones. The i-th stone from the left is white if the i-th character of S is ., and the stone is black if the character is #.

Takahashi wants to change the colors of some stones to black or white so that there will be no white stone immediately to the right of a black stone.
Find the minimum number of stones that needs to be recolored.

Constraints

1 \leq N \leq 2\times 10^5

S is a string of length N consisting of . and #.

Input

Input is given from Standard Input in the following format:

N
S

Output

Print the minimum number of stones that needs to be recolored.

Sample Input 1

3
#.#

Sample Output 1

1

It is enough to change the color of the first stone to white.

Sample Input 2

5
#.##.

Sample Output 2

2

Sample Input 3

9
.........

Sample Output 3

0

Solution in C# (Mono 4.6.2.0) (judge verdict: Wrong Answer)

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

class Program
{
    static void Main(string[] args)
    {
        //黒から白に変わるポイントを消すための対処法は次の二通り。

        //最初に黒から白に変わるポイント以降を全部黒にする
        //最後に黒から白に変わるポイント以前を全部白にする

        int N = int.Parse(Console.ReadLine());
        string S = Console.ReadLine();

        if (S == "#" || S == ".")
        {
            Console.WriteLine(0);
            return;
        }
        

        //それより左に何個黒があるかを知る配列。
        int[] arr1 = new int[N];
        //それより左に何個白があるかを知る配列
        int[] arr2 = new int[N];

        int siroNum = 0;
        int kuroNum = 0;
        for (int i = 0; i < N; i++)
        {
            char c = S[i];
            if (c == '#') kuroNum++;
            else if (c == '.') siroNum++;

            arr1[i] = kuroNum;
            arr2[i] = siroNum;
        }

        int ans = int.MaxValue;
        for (int i = 0; i < N; i++)
        {
            //i以前を全部白にしてiより後を全部黒にすると仮定するとどれだけ
            int n1 = arr1[i];
            int n2 = arr2[N - 1] - arr2[i];
            ans = Math.Min(ans, n1 + n2);
        }

        Console.WriteLine(ans);
    }
}