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 TenkaPBC2019C2
{
    class Program
    {
        static void Main(string[] args)
        {
            int N = int.Parse(Console.ReadLine());//石の数Ｎ個
            string S = Console.ReadLine();//文字列　．白　＃黒

            int black = 0;
            int white = S.Replace("#", "").Length;//.の数を数えてみる
            int ans = white;
            if ((white == 0) || (white == N))//.がひとつもない、または全部.なら何もしなくてよい。
            {
                Console.WriteLine(0);
            }
            else
            {
                for (int i = 0; i < N; i++)
                {
                    int counter = 0;
                    if (S.Substring(i, 1) == "#")
                    {
                        black++;
                    }
                    else
                    {
                        white--;
                    }
                    counter = black + white;
                    if (counter < ans)
                    {
                        ans = counter;
                    }
                }
                Console.WriteLine(ans);
            }
            
        }
    }
}