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;

class Program
{
    static void Main(string[] args)
    {
        string input = Console.ReadLine();
        int N = int.Parse(input);
        input = Console.ReadLine();
        char[] S= input.ToCharArray();
        int white_all = 0;
        int black = 0;
        for(int i = 0; i < N; i++)
        {
            if (S[i] == '.') { white_all++; }
        }
        int ans = white_all;
        int counter = 0;
        int white = white_all;
        if (white_all == S.Length || white_all == 0) ans = 0;
        else
        {
            for (int i = 0; i < N; i++)
            {
                if (S[i] == '#') { black++; }
                else { white--; }
                counter = black + white;
                if (counter < ans) { ans = counter; }
            }
        }
        Console.WriteLine(ans);
    }
}