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)

from copy import copy

def rewrite(S,c):
    output = 0
    for i in range(1,len(S)):
        if S[i-1] == "B" and S[i] == "W":
            if c == "B":
                S[i-1] = "W"
            else:
                S[i] = "B"
            output += 1
    return output

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

b,w = 0,0
for i in range(N):
    if S[i] == ".":
        S[i] = "W"
        w += 1
    else:
        S[i] = "B"
        b += 1

S1,S2 = copy(S),copy(S)
print(min(rewrite(S1,"W"),rewrite(S2,"B")))