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;
using System.Linq;
using System.Collections.Generic;
using static System.Console;
using System.Text;
using System.IO;
using static System.Math;
namespace AtCoder
{
    class Program
    {
        static public long[] Sarray() { return ReadLine().Split().Select(long.Parse).ToArray(); }
        static public List<long> Slist() { return ReadLine().Split().Select(long.Parse).ToList(); }

        static long Mod = (long)1e9 + 7;
        static long[] dirw = new long[] { -1, 0, 1, 0 };
        static long[] dirh = new long[] { 0, 1, 0, -1 };
        static string[] Sij;

        static void Main(string[] args)
        {
            //SetOut(new StreamWriter(OpenStandardOutput()) { AutoFlush = false });
            var N = Sarray()[0];
            var S = ReadLine();
            // 黒確定以降は全部黒になる。iで黒を確定させるときの値と全部白で行く場合を考える
            // dp[0]:黒確定
            // dp[1]:白のまま
            var dp = new long[2];
            for (var i = 0; i < S.Length; ++i)
            {
                if (S[i] == '#')//黒
                {
                    dp[0] = Min(dp[0],dp[1]);// 黒を確定させる
                    dp[1] = dp[1] + 1;
                }
                else// 白
                {
                    dp[0] = dp[0] + 1;
                    dp[1] = dp[1];
                }
            }
            WriteLine(Min(dp[0], dp[1]));
            //Out.Flush();
        }
    }
}