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

cntB = 0
cntW = 0

left  = 0
right = n - 1

s.each do |c|
  if c == '.'
    cntW += 1
  else
    left = cntW
    break
  end
end

s.reverse.each do |c|
  if c == '#'
    cntB += 1
  else
    right = n - cntB - 1
    break
  end
end

if 0 <= left && right <= n - 1 && left < right
  ary = s[left..right]
  min = 1E6

  cntB = 0
  cntW = ary.count('.')

  ary.each do |a|
    if a == '#'
      min = [min, cntB + cntW].min
      cntB += 1
    else
      cntW -= 1
    end
  end

  puts min
else
  puts 0
end