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)

# coding: utf-8
# Your code here!

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

white = S.count('.')
black = N - white

l_white = 0
l_black = 0
r_white = white
r_black = black

cost = l_black + r_white

for s in S:
    if s == '.':
        l_white += 1
        r_white -= 1
    else:
        l_black += 1
        r_black -= 1
    cost = min(cost, l_black + r_white)
    
print(cost)