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)

# frozen_string_literal: true

n = gets.chomp.to_i
s = gets.chomp.chars

# もういいや 最初に###が最後でおわるかどうか判断するわ
endbyblack = true
firstarrablack = s[-1] == '#' ? 1 : 0
(0...n - 1).to_a.each do |i|
  endbyblack = false if s[i] == '#' && s[i + 1] == '.'
  firstarrablack += 1 if s[i] == '#' && endbyblack == true
end

if endbyblack
  puts 0
  exit
end

w = 0
b = s[0] == '#' ? 1 : 0
whiteon = false
(1...n).to_a.each do |i|
  if whiteon && s[i] == '.'
    w += 1
  elsif s[i - 1] == '#' && s[i] == '.'
    w += 1
    whiteon = true
  elsif s[i] == '#'
    b += 1
  end
end
p firstarrablack
puts [w, b, firstarrablack].min