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: Wrong Answer)

def main():
    n = int(input())
    s = list(input())

    buff = s[0]

    while_count = 0

    black_count = 0

    for stone in s:
        if buff == "#" and stone == ".":
            while_count += 1

        else:
            buff = stone

    buff = s[n-1]

    for stone in s[::-1]:
        if buff == "." and stone == "#":
            black_count += 1

        else:
            buff = stone

    black_stone = "".join(s).count("#")

    print(min(while_count, black_count, black_stone, n - black_stone))


if __name__ == "__main__":
    main()