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)

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

black_count = s.count('#')
white_count = s.count('.')
if black_count == 0 || white_count == 0
  puts 0
  exit
end

first_index = s.index('.#')
if first_index == nil
  black_count = s.count('#')
  white_count = s.count('.')
  count = black_count > white_count ? white_count : black_count
  puts count
  exit
end
first_count = s[0...first_index].count('#') + s[(first_index+1)...n].count('.')

index = first_index + 1
previous_count = first_count
min_count = first_count
while true do
  black_index = s.index('.#', index)
  break if black_index == nil
  substr = s[index..black_index]
  count = previous_count + substr.count('#') - substr.count('.')

  if count < min_count
    min_count = count
  end

  index = black_index + 1
  previous_count = count
end
puts min_count