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)

import sys

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

score1 = 0
score2 = 0
length = 1
check = S[0]
A = []
B = []
flag = 0

for i in range(N-1):
    if S[i] == S[i+1]:
        length += 1
    else:
        A.append(length)
        length = 1

if length != 1:
    A.append(length)

if len(A) != 0:
    if S[-1] != S[-2]:
        A.append(1)

if len(A) <= 1:
    print(0)
    sys.exit()

if S[0] == ".":
    A.pop(0)

if S[len(S)-1] == "#":
    A.pop(-1)

if len(A) == 0:
    print(0)
    sys.exit()

for i in range(len(A)):
    if i % 2 == 0:
        score1 += A[i]
    else:
        score2 += A[i]

ans = min(score1,score2)

if S[0] == "#":
    score3 = A[0] + score2 - A[1]
    ans = min(ans,score3)

if S[0] == ".":
    score4 = A[len(A)-1] + score1 - A[len(A)-2]
    ans = min(ans,score4)

print(ans)