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: Runtime Error)

#C - Stones
#黒い石のすぐ右に白い石があるような箇所がないようにしたいです。 

n = int(input())
state = input()
state_list = list(state)
state_list.append(" ")

#specify the locate causing problem
prob_locate = []
prob_num = 0
for i in range(n):
	if state_list[i] == "#" and state_list[i+1] == ".":
		prob_locate.append(i)
		prob_num += 1
if prob_num == 0:
		print(0)
		exit()

#finding optimal position
import random
while prob_num > 0:
	for i in prob_locate:
		r_0 = random.randint(0,1)
		r_1 = random.randint(0,1)
		state_list[(i + r_0)] = ["#", "."][r_1]
	prob_locate = []
	prob_num = 0
	for i in range(n):
		if state_list[i] == "#" and state_list[i+1] == ".":
			prob_locate.append(i)
			prob_num += 1

#checking the difference
statelist.remove(" ")
diff = 0
for i, j in zip(state_list, list(state):
	if i != j: diff += 1

print(diff)