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()
start = s[0]
pattern = []

prev = ''
b = 0
w = 0
for i in s:
    if (i == '#' and prev == '#') or (i == '#' and prev ==''):
        b += 1
    elif (i == '.' and prev == '.') or (i == '.' and prev == ''):
        w += 1
    elif i == '#':
        b += 1
        pattern.append(w)
        w = 0
    else:
        w += 1
        pattern.append(b)
        b = 0
    prev = i
    
if prev == '#':
    pattern.append(b)
else:
    pattern.append(w)

totalb = 0
totalw = 0
for i in range(len(pattern)):
    if start == '.':
        if i % 2 == 0:
            totalw += pattern[i]
        else:
            totalb += pattern[i]
    else:
        if i % 2 == 0:
            totalb += pattern[i]
        else:
            totalw += pattern[i]
        
if len(pattern) == 1 or (len(pattern) == 2 and start == '.'):
    print(0)
elif (len(pattern) == 2 and start == '#'):
    print(min(totalw, totalb))
else:
    print(min(totalw - pattern[0], totalb - pattern[-1]))