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 Rust (1.15.1) (judge verdict: Accepted)

use std::io::*;
use std::str::FromStr;

#[allow(unused_macros)]
macro_rules! scan {
  ($e:expr; $t:ty) => {
    $e.get::<$t>()
  };
  ($e:expr; $($t:ty), *) => {
    ($($e.get::<$t>(),)*)
  }
}

struct Scanner<R: BufRead> {
  reader: R,
  iter: std::vec::IntoIter<String>
}

#[allow(dead_code)]
impl<R: BufRead> Scanner<R> {
  fn new(reader: R) -> Scanner<R> {
    Scanner { reader: reader, iter: vec![String::new()].into_iter() }
  }
  fn new_line(&mut self) {
    let mut line = String::new();
    self.reader.read_line(&mut line).ok();
    self.iter = line.trim().split_whitespace().map(|s| s.to_string()).collect::<Vec<String>>().into_iter();
  }
  fn get<T: FromStr>(&mut self) -> T {
    self.iter.next().unwrap().parse().ok().unwrap()
  }
  fn get_as_vec<T: FromStr>(&mut self) -> Vec<T> {
    self.iter.clone().map(|v| v.parse().ok().unwrap()).collect()
  }
  fn get_line(&mut self) -> String {
    let mut line = String::new();
    self.reader.read_line(&mut line).ok();
    line.trim().to_string()
  }
}

fn main() {
  let cin = stdin();
  let cin = cin.lock();
  let mut sc = Scanner::new(cin);
  sc.new_line();
  let _ = scan!(sc; i64);
  let s = sc.get_line();
  let mut ans = std::usize::MAX;
  if s.chars().all(|c| c == '.') || s.chars().all(|c| c == '#') {
    println!("0");
    return;
  }
  let mut wr = s.chars().filter(|&c| c == '.').collect::<String>().len();
  let mut br = s.chars().filter(|&c| c == '#').collect::<String>().len();
  let wn = wr;
  let bn = br;
  let mut bl = 0;
  for c in s.chars() {
    match c {
      '.' => { wr -= 1; },
      '#' => { br -= 1; bl += 1; },
      _   => {}
    }
    ans = if ans < bl + wr { ans } else { bl + wr };
  }
  ans = if ans > wn { wn } else if ans > bn { bn } else { ans };
  println!("{}", ans);
}