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 Python (3.4.3) (judge verdict: Accepted)

import sys
input = sys.stdin.readline

N = int(input())
S = input()

left_w = [0] * (N)
right_b = [0] * (N)

for i in range(N):
  if i == 0:
    left_w[0] = int(S[0] == '#')
    right_b[-1] = int(S[-1] == '.')
  else:
    left_w[i] = left_w[i-1] + int(S[i] == '#')
    right_b[-i-1] = right_b[-i] + int(S[-i-1] == '.')
left_w = [0] + left_w
right_b = right_b + [0]
# print(left_w)
# print(right_b)

res = N
for i in range(N+1):
  res = min(res, right_b[i] + left_w[i])
print(res)