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.reverse

run = []

right = false
cur = "."
count = 0

if S[0] == "#"
  right = true
  cur = "#"
end

for i in 0..N-1 do
  if S[i] == cur
    count += 1
  else
    run.push(count)
    count = 1
    if cur == "."
      cur = "#"
    else
      cur = "."
    end
  end
end

run.push(count)
l = run.length

min = 0
tmp = 0
all0 = 0
all1 = 0

#右端が1'#'のとき
if right
  if l != 2
    for i in 0..l-2 do
      if i.even?
        all1 += run[i]
      else
        all0 += run[i]
      end

      if (l-1).odd?
        all0 += run[l-1]
      end
    end
      min = [all0, all1].min
  end
else
  for i in 0..l-2 do
    if i.odd?
      all0 += run[i]
    else
      all1 += run[i]
    end

    if (l-1).odd?
      all0 += run[l-1]
    end
    min = [all0, all1].min
  end
end

puts min