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;

public class Program{
    public static void Main(){
        
        var n = int.Parse(Console.ReadLine());
        
        var s = Console.ReadLine();
        
        int[] cnt = new int[2];
        for(var i = 0; i < n; i++){
            if(s[i] =='.'){
                cnt[1]++;
            }
        }
        var min = cnt.Sum();
        
        //border:左側の右端の値
        for(var border = 0; border < n; border++){
            if(s[border] == '#'){
                cnt[0]++;
            }
            if(s[border] == '.'){
                cnt[1]--;
            }
            if(min > cnt.Sum()){
                min = cnt.Sum();
            }
        }
        
        Console.WriteLine("{0}",min);
    }
}