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: Wrong Answer)

using System;

namespace Tenka1BC2019_C
{
    class Program
    {
        static void Main(string[] args)
        {
            var N = int.Parse(Console.ReadLine());
            var S = Console.ReadLine();

            var ans = 0;
            var cur = S[0];
            var cntB = S[0] == '#' ? 1 : 0;
            var cntW = S[0] == '.' ? 1 : 0;
            for (int i = 1; i < S.Length; i++)
            {
                if (S[i] == cur && cur == '#') cntB++;
                if (S[i] == cur && cur == '.') cntW++;

                if (S[i] != cur && cur == '#')
                {
                    cur = '.';
                    cntW++;
                }
                else if (S[i] != cur && cur == '.')
                {
                    ans += Math.Min(cntB, cntW);
                    cntW = 0;
                    cntB = 1;
                    cur = '#';
                }
            }

            if (S[S.Length - 1] == '.')
            {
                ans += Math.Min(cntB, cntW);
            }

            Console.WriteLine(ans);
        }
    }
}