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: Wrong Answer)

package main

import (
	"bufio"
	"fmt"
	"log"
	"math"
	"os"
	"strconv"
)

func main() {
	n := nextInt()
	s := nextString()
	s1 := []byte(s)
	s2 := []byte(s)
	cnt1 := 0
	cnt2 := 0
	for i := 0; i < n-1; i++ {
		if s1[i] == '#' && s1[i+1] == '.' {
			s1[i] = '.'
			cnt1++
			for j := i - 1; j >= 0; j-- {
				if s1[j] == '#' && s1[j+1] == '.' {
					s1[j] = '.'
					cnt1++
				} else {
					break
				}
			}
		}
	}
	for i := 0; i < n-1; i++ {
		if s2[i] == '#' && s2[i+1] == '.' {
			s2[i+1] = '#'
			cnt2++
		}
	}
	log.Println(cnt1, string(s1))
	log.Println(cnt2, string(s2))
	fmt.Println(min(cnt1, cnt2))
}

var dy = []int{1, 0, -1, 0, 0}
var dx = []int{0, 1, 0, -1, 0}
var inf = int(1e9)

func pow(a, b int) int {
	return int(math.Pow(float64(a), float64(b)))
}

func lcm(a, b int) int {
	return a * b / gcd(a, b)
}
func gcd(a, b int) int {
	if a < b {
		a, b = b, a
	}
	for b != 0 {
		a, b = b, a%b
	}
	return a
}
func max(a ...int) int {
	r := a[0]
	for i := 0; i < len(a); i++ {
		if r < a[i] {
			r = a[i]
		}
	}
	return r
}
func min(a ...int) int {
	r := a[0]
	for i := 0; i < len(a); i++ {
		if r > a[i] {
			r = a[i]
		}
	}
	return r
}
func sum(a []int) (r int) {
	for i := range a {
		r += a[i]
	}
	return r
}
func abs(a int) int {
	if a < 0 {
		return -a
	}
	return a
}

type pair struct {
	a, b int
}

type pairs []pair

func (p pairs) Len() int {
	return len(p)
}
func (p pairs) Swap(i, j int) {
	p[i], p[j] = p[j], p[i]
}

func (p pairs) Less(i, j int) bool {
	if p[i].a == p[j].a {
		return p[i].b < p[j].b
	}
	return p[i].a < p[j].a
}

var nextReader func() []byte

func init() {
	nextReader = newScanner()
	log.SetFlags(log.Lshortfile)
}

func newScanner() func() []byte {
	r := bufio.NewScanner(os.Stdin)
	r.Buffer(make([]byte, 10000), int(1000000))
	r.Split(bufio.ScanWords)
	return func() []byte {
		r.Scan()
		return r.Bytes()
	}
}
func nextBytes() []byte {
	return nextReader()
}

func nextString() string {
	return string(nextReader())
}
func nextInt64() int64 {
	v, _ := strconv.ParseInt(string(nextReader()), 10, 64)
	return v
}
func nextInt() int {
	v, _ := strconv.Atoi(string(nextReader()))
	return v
}
func nextInts(n int) []int {
	r := make([]int, n)
	for i := 0; i < n; i++ {
		r[i] = nextInt()
	}
	return r
}
func nextFloat64() float64 {
	f, _ := strconv.ParseFloat(string(nextReader()), 64)
	return f
}