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.Linq;
using System.Collections.Generic;

class Program
{

    static void Main(string[] args)
    {
        var N = int.Parse(Console.ReadLine());
        var S = Console.ReadLine();

        var firstBlackIndex = -1;
        // 最初の黒をみつける
        for (int i = 0; i < N; i++)
        {
            if (S[i] == '#')
            {
                firstBlackIndex = i;
                break;
            }
        }

        // 黒を見つけた後、右に白がある場合は黒にすればよい
        int count = 0;
        for (int i = firstBlackIndex + 1; firstBlackIndex >= 0 && i < N; i++)
        {
            if (S[i] == '.')
            {
                count++;
            }
        }

        // 最初に白をみつける
        int lastWhiteIndex = -1;
        for (int i = N - 1; i >= 0; i--)
        {
            if (S[i] == '.')
            {
                lastWhiteIndex = i;
                break;
            }
        }

        int wCount = 0;
        for (int i = lastWhiteIndex - 1; lastWhiteIndex >= 0 && i >= 0; i--)
        {
            if (S[i] == '#')
            {
                wCount++;
            }
        }

        int minCount = Math.Min(count, wCount);

        Console.WriteLine(minCount);
    }

}