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 itertools
from collections import Counter
import bisect

def main():
    N = int(input())
    S = list(input())
    # print(S)

    if S.count('.') == N or S.count('#') == N:
        print(0)
        exit()

    total_white = S.count('.')
    ans = total_white
    rest_white = total_white

    current_ans = 0
    S += '#'  # 番兵
    black_count = 0
    white_count = 0
    for i in range(len(S)):
        if S[i] == '#':
            black_count += 1
            if white_count > 0:
                # consider
                ans = min(ans, current_ans + white_count + rest_white)
                current_ans += (black_count - 1)
                white_count = 0
                black_count = 1
        else:
            if black_count > 0:
                white_count += 1
            # print('w')
            rest_white -= 1
    print(ans)

if __name__ == '__main__':
    main()