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 AtCoder {

    class Program {

        static void Main(string[] args) {

            var N = int.Parse(Console.ReadLine());
            var originalS = Console.ReadLine().ToCharArray();
            var S = new string(originalS).ToCharArray();

            var result = 0;
            int i = 0;
            foreach(var s in S) {

                if(s == '#') {

                    if(i + 1 >= N) {

                        break;
                    }
                    if(S[i + 1] == '.') {

                        S[i + 1] = '#';
                        result++;
                    }
                }
                i++;
            }

            S = new string(originalS).ToCharArray();
            var result2 = 0;
            i = 0;
            foreach (var s in S) {

                if (s == '#') {

                    if (i + 1 >= N) {

                        break;
                    }
                    if (S[i + 1] == '.') {

                        S[i] = '.';
                        result2++;
                    }
                }
                i++;
            }
            Console.WriteLine(Math.Min(result, result2));
        }
    }
}