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)

N = int(input())
S = input()
 
sm_w = [0]*(N+1)
sm_b = [0]*(N+1)
 
for i in range(N):
    s = S[i]
    sm_w[i+1] += sm_w[i]
    sm_b[i+1] += sm_b[i]
    if s == '.':
        sm_w[i+1] += 1
    else:
        sm_b[i+1] += 1
 
ans = 2**60
for i in range(N+1):
    left_b = 0
    if i!=0:
        left_b = sm_b[i]
    right_w = 0
    if i!=N:
        right_w = sm_w[N] - sm_w[i]
    ret = left_b + right_w
    ans = min(ans, ret)
print(ans)