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)

n = int(input())
s = input()

def calc():
    min_cost = n
    left_cost = 0
    right_cost = s.count('.')

    if right_cost == 0:
        return(0)

    for i in range(n):
        if s[i] == '#':
            left_cost += 1
        else:
            right_cost -= 1
        cost = left_cost + right_cost
        if cost < min_cost:
            min_cost = cost

    return(min_cost)

print(calc())