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;

namespace C
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            string s = Console.ReadLine();
            int w = 0;
            int wlast = -1;
            int b = 0;
            int bfirst = -1;
            int ans = 100100100;
            for (int i = 0; i < n; i++)
            {
                if (s[i] == '.')
                {
                    w++;
                    wlast = i;
                }
                if (s[i] == '#')
                {
                    b++;
                    if (bfirst == -1) bfirst = i;
                }
            }
            //全部白にする
            ans = (int)Math.Min(ans, b);
            //全部黒
            ans = (int)Math.Min(ans, w);
            //左詰め白
            if (w > 0)
            {
                int cnt = 0;
                for (int i = 0; i <= wlast; i++)
                {
                    if (s[i] == '#') cnt++;
                }
                ans = (int)Math.Min(ans, cnt);
            }
            //右詰め黒
            if (b > 0)
            {
                int cnt = 0;
                for (int i = bfirst; i <n; i++)
                {
                    if (s[i] == '.') cnt++;
                }
                ans = (int)Math.Min(ans, cnt);
            }
            Console.WriteLine(ans);
        }
    }
}

//namespace C
//{
//    class Program
//    {
//        static void Main(string[] args)
//        {
//            int n = int.Parse(Console.ReadLine());
//            string s = Console.ReadLine();
//            int w = 0;
//            int b = 0;
//            for(int i = 0; i < n; i++)
//            {
//                if (s[i] == '.') w++;
//                if (s[i] == '#') b++;
//            }
//            int ans = 0;
//            if (w >= b)
//            {
//                ans = b;
//                int cnt = n-1;
//                while (s[cnt] == '#')
//                {
//                    ans--;
//                    cnt--;
//                }
//            }else if (w < b)
//            {
//                ans = w;
//                int cnt = 0;
//                while (s[cnt] == '.')
//                {
//                    ans--;
//                    cnt++;
//                }
//            }
//            Console.WriteLine(ans);
//        }
//    }
//}