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.Diagnostics;

class Program
{
    static void Main(string[] args)
    {
        int n = int.Parse(Console.ReadLine());
        string s = Console.ReadLine();

        int[] sharp = new int[n];
        int[] dot = new int[n];

        if (s[0] == '#') sharp[0] = 1;
        else sharp[0] = 0;
        for (int i = 1; i < n; i++)
        {
            if (s[i] == '#') sharp[i] = sharp[i - 1] + 1;
            else sharp[i] = sharp[i - 1];
        }
        dot[n - 1] = 0;
        for (int i = n - 2; 0 <= i; i--)
        {
            if (s[i + 1] == '.') dot[i] = dot[i + 1] + 1;
            else dot[i] = dot[i + 1];
        }

        int ans = int.MaxValue;
        for (int i = 0; i < n; i++)
        {
            ans = Math.Min(ans, sharp[i] + dot[i]);
        }

        if (s[0] == '.') ans = Math.Min(ans, dot[0] + 1);
        else ans = Math.Min(ans, dot[0]);
        Console.WriteLine(ans);
    }
}