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: Wrong Answer)

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

public class AtCoderA
{
    static void Main()
    {
        var sc = new Scanner();

        int n = sc.NextInt();
        string s = sc.Next();

        long[] leftBlackSum = new long[n];
        leftBlackSum[0] = s[0] == '#' ? 1 : 0;
        for (int i = 1; i < n; i++)
        {
            leftBlackSum[i] = leftBlackSum[i - 1] + (s[0] == '#' ? 1 : 0);
        }

        long[] rightWhiteSum = new long[n];
        rightWhiteSum[n - 1] = s[n - 1] == '.' ? 1 : 0;
        for (int i = 1; i < n; i++)
        {
            rightWhiteSum[n - 1 - i] = rightWhiteSum[n - i] + (s[n - 1 - i] == '.' ? 1 : 0);
        }

        long answer = long.MaxValue;

        for (int i = 0; i < n; i++)
        {
            long tmpAnswer = (i == 0 ? 0 : leftBlackSum[i - 1]) + rightWhiteSum[i];
            answer = Math.Min(answer, tmpAnswer);
        }
        answer = Math.Min(answer, leftBlackSum[n - 1]);

        Console.WriteLine(answer);
    }

    class Scanner
    {
        private string[] _line;
        private int _index;
        private const char Separator = ' ';

        public Scanner()
        {
            _line = new string[0];
            _index = 0;
        }

        public string Next()
        {
            while (_index >= _line.Length)
            {
                _line = Console.ReadLine().Split(Separator);
                _index = 0;
            }

            return _line[_index++];
        }

        public int NextInt() => int.Parse(Next());
        public long NextLong() => long.Parse(Next());
        public double NextDouble() => double.Parse(Next());
        public decimal NextDecimal() => decimal.Parse(Next());
        public char NextChar() => Next()[0];
        public char[] NextCharArray() => Next().ToCharArray();

        public string[] Array()
        {
            _line = Console.ReadLine().Split(Separator);
            _index = _line.Length;
            return _line;
        }

        public int[] IntArray() => Array().Select(int.Parse).ToArray();
        public long[] LongArray() => Array().Select(long.Parse).ToArray();
        public double[] DoubleArray() => Array().Select(double.Parse).ToArray();
        public decimal[] DecimalArray() => Array().Select(decimal.Parse).ToArray();
    }
}