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 Kotlin (1.0.0) (judge verdict: Accepted)

fun main(args: Array<String>) {
    val n = readLine()!!.toInt()
    val s = readLine()!!
    var ans = s.count { it == '.' }
    ans = Math.min(ans, s.count { it == '#' })
    val blackCount = Array(n) { 0 }
    blackCount[0] = if (s[0] == '#') 1 else 0
    (1 until n).forEach { blackCount[it] = blackCount[it - 1] + if (s[it] == '#') 1 else 0 }

    val whiteCount = Array(n) { 0 }
    whiteCount[n - 1] = if (s[n - 1] == '.') 1 else 0
    (n - 2 downTo 0).forEach { whiteCount[it] = whiteCount[it + 1] + if (s[it] == '.') 1 else 0 }

    for (i in 0 until (n - 1)) {
        val white = blackCount[i]
        val black = whiteCount[i + 1]
        ans = Math.min(ans, white + black)
    }
    println(ans)
}