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)

// Try Tenka1 Programmer Contest 2019
// author: Leonardone @ NEETSDKASU

fn main() {
	let mut stdin = String::new();
    std::io::Read::read_to_string(
    	&mut std::io::stdin(),
        &mut stdin).unwrap();
	let mut stdin = stdin.split_whitespace();
    let mut get = || stdin.next().unwrap();
    macro_rules! get {
    	($t:ty) => (get().parse::<$t>().unwrap());
        () => (get!(i64));
    }
    
    let n = get!(usize);
    let s = get().as_bytes();
    let mut sumw = vec![0; n+1];
    let mut sumb = vec![0; n+1];
    for i in 0..s.len() {
    	sumw[i+1] = sumw[i];
        sumb[i+1] = sumb[i];
    	if s[i] == b'.' {
        	sumw[i+1] += 1;
        } else {
        	sumb[i+1] += 1;
        }
    }
    let mut ans = n;
    for i in 0..n+1 {
    	let tmp = sumb[i]+sumw[n]-sumw[i];
        ans = std::cmp::min(ans, tmp);
    }
    println!("{}", ans);
}