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;

namespace Tenka2019
{
    class ProgramC
    {
        static void Main(string[] args)
        {
            var n = int.Parse(Console.ReadLine());
            var s = Console.ReadLine();

            var bL2R = new int[n];
            var wR2L = new int[n];

            var tmp = 0;
            for (var i = 0; i < n; i++)
            {
                if (s[i] == '#') // Black
                {
                    tmp++;
                }
                bL2R[i] = tmp;
            }

            tmp = 0;
            for (var i = 0; i < n; i++)
            {
                if (s[n - i - 1] == '.') // white
                {
                    tmp++;
                }
                wR2L[i] = tmp;
            }

            var minCount = wR2L[n - 1];
            for (var i = 0; i < n - 1; i++)
            {
                var count = bL2R[i] + wR2L[n - i - 2];
                minCount = Math.Min(minCount, count);
            }

            minCount = Math.Min(minCount, bL2R[n-1]);

            Console.WriteLine(minCount);
        }
    }
}