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 Java8 (OpenJDK 1.8.0) (judge verdict: Accepted)

import java.util.*;
import java.io.*;

import static java.lang.System.in;

public class Main {
    public static void main(String[] args) throws FileNotFoundException {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        char[] s = sc.next().toCharArray();
        int[] bLeft = new int[n+2], wRight = new int[n+2];
        for(int i=0;i<n;i++) bLeft[i+1] = bLeft[i]+(s[i]=='#'?1:0);
        for(int i=n-1;i>=0;i--) wRight[i+1] = wRight[i+2]+(s[i]=='.'?1:0);
        int ans = n;
        for(int i=1;i<=n;i++){
            int cur = bLeft[i-1]+wRight[i];
            ans = Math.min(ans,cur);
        }
        ans = Math.min(ans,bLeft[n]);
        System.out.println(ans);
    }
}