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)

# 完成形から考える問題
n = gets.chomp.to_i
s = gets.chomp.chars

black_count = 0

s.each { |val| black_count += 1 if val == "#" }

left_b = 0
left_w = 0
right_b = black_count
right_w = n - black_count

ans = left_b + right_w


n.times do |i|

  if s[i] == '#'
    left_b += 1
    right_b -= 1
  else
    left_w += 1
    right_w -= 1
  end
  ans = [ans, left_b + right_w].min
end

p ans