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.Linq;
using static System.Math;
using static System.Console;

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

        char cOld = s[0];
        n = s.Length;

        var wCount = 0;
        var bCount = 0;
        var blist = new int[n + 1];
        var wlist = new int[n + 1];

        for (int i = 0; i < s.Length; i++)
        {
            if (s[i] == '#')
            {
                wCount++;
            }
            if (s[n - 1 - i] == '.')
            {
                bCount++;
            }
            wlist[i + 1] += wCount;
            blist[n - 1 - i] += bCount;
        }
        WriteLine(wlist.Zip(blist, (w, b) => w + b).Min());
    }
}