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 Go (1.6) (judge verdict: Accepted)

package main

import (
	"fmt"
)

func min(a, b int) int {
	if a < b {
		return a
	} else {
		return b
	}
}

func main() {
	var n int
	var s string

	fmt.Scanf("%d", &n)
	fmt.Scanf("%s", &s)

	if n <= 1 {
		fmt.Println(0)
		return
	}

	black := make([]int, n)
	white := make([]int, n)

	black[0] = 0
	white[0] = 0

	bByte := "#"[0]

	ans := n

	for i := 0; i < n; i++ {
		prev := i - 1
		if i <= 0 {
			prev = 0
		}
		if s[i] == bByte {
			black[i] = black[prev] + 1
			white[i] = white[prev]
		} else {
			white[i] = white[prev] + 1
			black[i] = black[prev]
		}
	}

	for i := 0; i < n; i++ {
		ans = min(ans, black[i]+(white[n-1]-white[i]))
	}
	ans = min(ans, black[n-1])
	ans = min(ans, white[n-1])

	fmt.Println(ans)
}