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 (3.8.2) (judge verdict: Wrong Answer)

N,S = open(0).read().split()
N=int(N)
S=list(S)

def makeGroup():
    group = [] if S[0] == '.' else [0]
    n = 0
    mark = S[0]
    for s in S:
        if s == mark:
            n += 1
        else:
            mark = s
            group.append(n)
            n = 1
    if n > 0:
        group.append(n)
    if mark == '#':
        group.append(0)
    return group

group = makeGroup()
i = 0
j = len(group) - 1
cnt = 0
while i < j:
    wl = group[i]
    bl = group[i+1]
    wr = group[j]
    br = group[j - 1]
    l = bl if wl <= bl else wl
    r = wr if wr <= br else br
    if l < r:
        cnt += l
        i += 2
    else:
        cnt += r
        j -= 2

print(cnt)