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


class Program
{
    static void Main(string[] args)
    {
        // 入力
        //var s = Console.ReadLine();
        var n = long.Parse(Console.ReadLine());
        char[] s = Console.ReadLine().ToCharArray();

        var ans = long.MaxValue;

        long[] b = new long[n];
        long[] w = new long[n];
        if (s[0] == '.') w[0] = 1;
        if (s[0] == '#') b[0] = 1;
        for (int i = 1; i < n; i++)
        {
            if (s[i] == '.') w[i] = w[i - 1] + 1;
            else w[i] = w[i - 1];

            if (s[i] == '#') b[i] = b[i - 1] + 1;
            else b[i] = b[i - 1];
        }

        ans = Math.Min(ans, w[n - 1]);
        ans = Math.Min(ans, b[n - 1]);
        for (int i = 0; i < n; i++)
        {
            var a = (b[n - i - 1]);
            var c = (w[n - 1] - w[n - i - 1]);
            ans = Math.Min(ans, (b[n - i - 1]) + (w[n - 1] - w[n - i - 1]));
        }


        // 出力
        Console.WriteLine(ans);
    }
}