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 static System.Console;
using static System.Math;

class Program
{
    static void Main()
    {
        var N = int.Parse(ReadLine());
        var S = ReadLine();

        var b = new int[N + 1];
        var w = new int[N + 1];
        for (int i = 0; i < N; i++)
        {
            b[i + 1] = b[i] + (S[i] == '#' ? 1 : 0);
        }
        for (int i = N - 1; i >= 0; i--)
        {
            w[i] = w[i + 1] + (S[i] == '.' ? 1 : 0);
        }

        var ans = int.MaxValue;
        for (int i = 0; i <= N; i++)
        {
            ans = Min(ans, b[i] + w[i]);
        }
        WriteLine(ans);
    }
}