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 Ruby (2.3.3) (judge verdict: Wrong Answer)

n = gets.to_i
s = gets.chomp.chars
_s = s.reverse
cnt_b = cnt_w = 0
(0..n-1).each do|i|
  if s[i] =="#" && s[i+1] == "."
    cnt_w += 1
    s[i+1] = "#"
  end
end

(0..n-1).each do|i|
  if _s[i] =="." && _s[i+1] == "#"
    cnt_b += 1
    _s[i+1] = "."
  end
end
puts [cnt_w,cnt_b].min