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: Accepted)

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

namespace TenkaBeginner2019C
{
    class Program
    {
        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());
            string S = Console.ReadLine();
            S += "#";

            //.の数の把握
            int countWhite = CountChar(S, '.');
            int countBlack = 0;

            int ans = int.MaxValue;
            for(int k = 0; k <= N; k++)
            {
                int ansCand = countWhite + countBlack;
                if (S[k] == '.')
                {
                    countWhite--;
                }
                else
                {
                    countBlack++;
                }
                if (ansCand < ans) ans = ansCand;
            }
            
            Console.WriteLine(ans);
            Console.ReadKey();
        }

        //static void Main(string[] args)
        //{
        //    int N = int.Parse(Console.ReadLine());
        //    string S = Console.ReadLine();
        //    S += "#";

        //    int ans = int.MaxValue;
        //    for (int k = 0; k <= N; k++)
        //    {
        //        int countW = CountChar(S.Substring(0, k).ToString(), '#');
        //        int countB = CountChar(S.Substring(k).ToString(), '.');
        //        int countSum = countW + countB;

        //        if (ans > countSum) ans = countSum;
        //    }

        //    Console.WriteLine(ans);
        //    Console.ReadKey();
        //}

        static int CountChar(string s, char c)
        {
            return s.Length - s.Replace(c.ToString(), "").Length;
        }
    }
}