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;
using System.Collections.Generic;
using System.Text;


public class C
{
	public static void Main()
	{
		int N = Int32.Parse(Console.ReadLine());
		string S = Console.ReadLine();
		
		int[] csumW = new int[N+1];
		int[] csumB = new int[N+1];
		csumW[0] = 0;
		csumB[0] = 0;
		for(int i = 1; i <= N; i++)
		{
			csumW[i] = csumW[i-1];
			csumB[i] = csumB[i-1];
			if(S[i-1] == '.')
				csumW[i]++;
			else
				csumB[i]++;
		}
		
		int ans = Int32.MaxValue;
		for(int i = 0; i <= N; i++)
		{
			// 1 to i : ....
			// i + 1 to N : ####
			int tmp = 0;
			tmp += csumB[i];
			tmp += (csumW[N] - csumW[i]);			
			ans = Math.Min(ans,tmp);		
		}
		
		Console.WriteLine(ans);
	}
}