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: Accepted)

def main():
	N = int(input())
	S = input()
	w = 0
	b = 0
	stack = []
	if S[0] == ".":
		stack.append(0)

	before = S[0]
	current = 0
	for i, c in enumerate(S):
		if c == ".":
			w += 1
		else:
			b += 1
		if c != before:
			stack.append(current)
			before = c
			current = 1
		else:
			current += 1
	stack.append(current)
	if len(stack) %2 != 0:
		stack.append(0)

	ans = min(w, b)
	w_change = 0
	b_change = b
	while stack:
		current_w = stack.pop()
		current_b = stack.pop()
		w_change += current_w
		b_change -= current_b
		ans = min(ans, w_change+b_change)

	print(ans)


if __name__ == '__main__':
    main()