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 static System.Math;
using System;

public class Hello
{
    public static void Main()
    {
        var n = int.Parse(Console.ReadLine().Trim());
        var s = Console.ReadLine().Trim();
        var w = new int[n];
        var b = new int[n];
        b[0] = s[0] == '#' ? 1 : 0;
        w[n - 1] = s[n - 1] == '.' ? 1 : 0;
        for (int i = 1; i < n; i++)
        {
            if (s[i] == '#') b[i] = b[i - 1] + 1;
            else b[i] = b[i - 1];
            var t = n - 1 - i;
            if (s[t] == '.') w[t] = w[t + 1] + 1;
            else w[t] = w[t + 1];
        }
        Console.WriteLine(getAns(b, w, s, n));
    }
    static int getAns(int[] b, int[] w, string s, int n)
    {
        var ans = int.MaxValue;
        for (int i = 0; i < n; i++)
            ans = Min(ans, w[i] + b[i] - 1);
        return ans;
    }
}