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 PHP (7.4.4) (judge verdict: Accepted)

<?php
list($n) = ints();
list($s) = strs();
$sinfo = substr_info($s);
$k = count($sinfo);
$cntw[0] = $cntb[0] = 0;
for ($i = 1; $i <= $k; ++$i) {
    $cntw[$i] = $cntw[$i - 1];
    $cntb[$i] = $cntb[$i - 1];
    if ($sinfo[$i - 1][0] === '.') $cntw[$i] += $sinfo[$i - 1][1];
    else $cntb[$i] += $sinfo[$i - 1][1];
}
$min = PHP_INT_MAX;
//...###
for ($i = 0; $i <= $k; ++$i)
    $min = min($min, $cntb[$i] + ($cntw[$k] - $cntw[$i]));
echo $min;
// 文字列の要素と連続数を格納した配列を返す
function substr_info(string $s): array
{
    $n = strlen($s);
    $cnt = 0;
    for ($i = 0; $i < $n; $i++) {
        $cnt++;
        if ($i === $n - 1 || $s[$i] !== $s[$i + 1]) {
            $_s[] = [$s[$i], $cnt];
            $cnt = 0;
        }
    }
    return $_s;
}

function strs()
{
    return explode(' ', trim(fgets(STDIN)));
}
function ints()
{
    return array_map('intval', explode(' ', trim(fgets(STDIN))));
}