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 PyPy3 (2.4.0) (judge verdict: Wrong Answer)

L = int(input())
S = input()

# . : white
# # : black
# no '#.' sequence

# a list of tuples of (start,end) for each color
white, black = [], []
nwhite, nblack = 0, 0

i, end = 0, 0
while(i<L):
    while(end<L):
        if S[i] == S[end]:
            end += 1
        else:
            break

    if S[i] == '.':
        white.append((i,end))
        nwhite += end-i
    else:
        black.append((i,end))
        nblack += end-i

    i = end

if nwhite == 0 or nblack == 0:
    print(0)

else:
    ans = 2 * (10 ** 5) + 1
    
    # set a index that starts sequence of black(#)
    for i, (candidate, curend) in enumerate(black):

        curans = 0

        # then turn over all black(#) left to it
        for (s,e) in black[:i]:
            curans = curans + e -s

        # then turn over all white(.) right to it    
        for (s,e) in white:
            if s > candidate:
                curans = curans + e -s 

        ans = min(ans, curans)

    print(ans)