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: Runtime Error)

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

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

            var list = new List<int>(n);
            int count = 1;
            for (int i = 1; i < n; i++)
            {
                if (s[i - 1] == s[i]) count++;
                else
                {
                    list.Add(count);
                    count = 1;
                }
            }
            list.Add(count);

            int[] sum = new int[4];
            if (s[0] == '.')
            {
                // white -> black
                for (int i = 2; i < list.Count; i += 2)
                    sum[0] += list[i];


                // black -> white
                for (int i = 1; i < list.Count; i += 2)
                    sum[1] += list[i];

                if (list.Count <= 2)
                {
                    if (list[0] < sum[0]) sum[0] = list[0];
                }
            }
            else
            {
                // white -> black
                for (int i = 1; i < list.Count; i += 2)
                    sum[0] += list[i];

                // black -> white
                for (int i = 0; i < list.Count; i += 2)
                    sum[1] += list[i];

                if (list.Count <= 3)
                {
                    if (list[0] < sum[1]) sum[1] = list[0];
                }
            }

            if (s[n - 1] == '.')
            {
                // black -> white
                for (int i = list.Count - 2; i >= 0; i -= 2)
                    sum[2] += list[i];

                // white -> black
                for (int i = list.Count - 1; i >= 0; i -= 2)
                    sum[3] += list[i];

                if (list.Count <= 3)
                {
                    if (list[n - 1] < sum[3]) sum[3] = list[n - 1];
                }
            }
            else
            {
                // black -> white
                for (int i = list.Count - 3; i >= 0; i -= 2)
                    sum[2] += list[i];

                // white -> black
                for (int i = list.Count - 2; i >= 0; i -= 2)
                    sum[3] += list[i];

                if (list.Count <= 2)
                {
                    if (list[n - 1] < sum[2]) sum[2] = list[n - 1];
                }
            }

            Console.WriteLine(sum.Min());
        }
    }
}