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# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Linq;

class MainClass
{
    static void Main()
    {
        int N = int.Parse(Console.ReadLine());
        string S = Console.ReadLine();

        int ans = int.MaxValue;
        int blackcount = S.Count(_ => _ == '#');
        int whitecount = S.Count(_ => _ == '.');

        int black = 0;
        int white = whitecount;
        ans = black + white;
        for (int i = 0; i < N; i++)
        {
            if (S[i] == '#') black++;
            else white--;
            ans = Math.Min(ans, black + white);
        }
        
        Console.WriteLine(ans);
    }
}