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)

// デバッグ実行：Ctrl+F5

using System;

class Program
{
    static void Main(string[] args)
    {
        String line;
        line = Console.ReadLine();
        int N = int.Parse(line);
        line = Console.ReadLine();
        string S = line;

        int MinCount = 200001;
        for(int i = 0; i < N; i++)
        {
            if (S[i] == '#')
            {
                //黒
                //白を全部黒にする or 黒を端っこ以外白にする
                int white_count = 0;
                for(int j = i + 1; j < N; j++)
                {
                    if (S[j] == '.') white_count++;
                }
                if (MinCount > white_count) MinCount = white_count;
                int black_count = N - i - white_count;
                for(int j = N-1; j >= i; j--)
                {
                    if (S[j] == '#') black_count--;
                    else break;
                }
                if (MinCount > black_count) MinCount = black_count;

                //現在地含めた黒の数
                black_count = N - i - white_count;
                //調べたところまでの黒の数（白にする必要のある数)
                int need_white = 0;
                for(int j = i; j < N; j++)
                {
                    if (S[j] == '.')
                    {
                        white_count--;
                    }
                    else
                    {
                        black_count--;
                        need_white++;
                    }
                    if (MinCount > white_count + need_white) MinCount = white_count + need_white;

                }
                break;
            }
            else
            {
                //白
                //無視
                if (i == N - 1)
                {
                    MinCount = 0;
                    break;
                }

            }
        }

        Console.WriteLine(MinCount);
    }
}