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: Accepted)

# coding: utf-8

N = gets.to_i
S = gets.chomp

a = S.split(/(?<=\.)(?=\#)|(?<=\#)(?=\.)/).map(&:size)

if S[0] == '.'
  a.unshift(0)
end
if S[-1] == '#'
  a.push(0)
end

→ = a.select.each_with_index {|_, i| i.even? }.inject([0]) do |memo, item|
  memo << memo.last + item
end
← = a.reverse.select.each_with_index {|_, i| i.even? }.inject([0]) do |memo, item|
  memo << memo.last + item
end
ans = →.zip(←.reverse).map do |ltr, rtl|
  ltr + rtl
end.min

puts ans