use std::io::{self, Write};

fn out(a: &[bool], k: usize) {
    for i in k - 1..a.len() {
        print!("{}", if a[i] { 1 } else { 0 });
    }
    println!();
}

fn to_bitarray(s: &str, s_arr: &mut Vec<bool>, k: usize, full_n: usize) {
    for i in 0..k - 1 {
        s_arr[i] = true;
    }
    for i in k - 1..full_n {
        s_arr[i] = s.chars().nth(i - k + 1).unwrap() == '1';
    }
}

fn min_find(s: &str, n: usize, full_n: usize) -> usize {
    let weight_min = (n + 1) / 2;
    let mut change = 0;
    let mut h_weight = 0;
    let mut s_arr = vec![false; full_n];
    let mut k_arr = vec![false; full_n];

    to_bitarray(s, &mut s_arr, weight_min, full_n);

    k_arr.copy_from_slice(&s_arr);

    for i in 0..n {
        h_weight += if s_arr[i] { 1 } else { 0 };
    }

    if h_weight < weight_min {
        s_arr[n - 1] = true;
        h_weight += 1;
        change += 1;
    }

    for i in 1..=full_n - n {
        h_weight -= if s_arr[i - 1] { 1 } else { 0 };
        h_weight += if s_arr[i + n - 1] { 1 } else { 0 };
        if h_weight < weight_min {
            s_arr[i + n - 1] = true;
            h_weight += 1;
            change += 1;
        }
    }

    change
}

fn main() {
    let mut input = String::new();
    io::stdout().flush().unwrap();
    io::stdin().read_line(&mut input).unwrap();
    let n: usize = input.trim().parse().unwrap();
    let mut s = String::new();
    io::stdin().read_line(&mut s).unwrap();
    println!("{}", min_find(&s.trim(), n, 3));
}