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.Text;
using System.Threading.Tasks;

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

            int[] white = new int[n];
            int[] black = new int[n];

            white[n - 1] = s[n - 1] == '.' ? 1 : 0;
            black[0] = s[0] == '#' ? 1 : 0;

            for (int i = 1; i < s.Length; i++)
            {
                black[i] = black[i - 1] + (s[i] == '#' ? 1 : 0);
            }
            for (int i = n - 2; i >= 0; i--)
            {
                white[i] = white[i + 1] + (s[i] == '.' ? 1 : 0);
            }

            int min = Math.Min(black[n - 1], white[0]);
            for (int i = 1; i < n; i++)
            {
                int next = white[i] + black[i - 1];
                min = Math.Min(next, min);
            }
            Console.WriteLine(min);
        }
    }
}