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.Arrays;
import java.util.Scanner;

public class Main {
	public void solve() {
		Scanner in = new Scanner(System.in);
		int N = in.nextInt();
		String stones = in.next();
		in.close();
		
		// i = [i-1] is left, [i] is right
		int[] left = new int[N+1];	// left is white(.)
		
		// i = [i-1] is left, [i] is right
		int[] right = new int[N+1];	// right is black(#)
		
		left[0] = 0;
		for (int i=0; i<N; i++) {
			if (stones.charAt(i) == '.') {
				left[i+1] = left[i];
			} else {
				left[i+1] = left[i] + 1;
			}
		}
		right[N] = 0;
		for (int i=N-1; i>=0; i--) {
			if (stones.charAt(i) == '#') {
				right[i] = right[i+1];
			} else {
				right[i] = right[i+1] + 1;
			}
		}
		
		int min = Integer.MAX_VALUE;
		for (int i=0; i<N+1; i++) {
			int val = left[i] + right[i];
			if (val < min) {
				min = val;
			}
		}
		
		System.out.println(min);
	}
	
	public static void main(String[] args) {
		Main main = new Main();
		main.solve();
	}
}