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;
class Tenka1_2019_C
{
    static void Main(string[] args)
    {
        int N = int.Parse(Console.ReadLine());
        string S = Console.ReadLine();

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

        int blackCount = 0;
        for (int i = 0; i < N; i++)
        {
            if(S[i]=='#') { blackCount++; }
            black[i] = blackCount;
            white[i] = (i+1) - blackCount;
        }

        if(black[N-1]==0 || white[N-1] == 0)
        {
            Console.WriteLine("0");
            return;
        }

        int min = int.MaxValue;
        for (int i = 0; i < N; i++)
        {
            if (S[i] == '.')
            {
                // 白
                if (black[N-1] < min) min = black[N-1];
            }
            else
            {
                // 黒
                int left = black[i] - 1;
                int right = white[N - 1] - white[i];

                if (left + right < min) min = left + right;
            }
        }

        Console.WriteLine(min);
    }
}