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 Scala (2.11.7) (judge verdict: Accepted)

import scala.io._
object Main extends App {
  val n = StdIn.readLine.toInt
  val s = StdIn.readLine

  def checkMin(i: Int, changes: Int, min: Int): Int = {
    if (i == n) min
    else {
      val newChanges = changes + (if (s.charAt(i) == '#') 1 else -1)
      checkMin(i + 1, newChanges, min.min(newChanges))
    }
  }
  val ws = s.count(_ == '.')
  println(checkMin(0, ws, ws))
}