fn main() {
    let stdin = std::io::read_to_string(std::io::stdin()).unwrap();
    let mut stdin = stdin.split_ascii_whitespace();

    let n: usize = stdin.next().unwrap().parse().unwrap();
    let a: Vec<u8> = (0..n)
        .map(|_| stdin.next().unwrap().parse().unwrap())
        .collect();

    use std::io::Write;
    std::io::stdout()
        .write_all(output(solve(a)).as_bytes())
        .unwrap();
}

fn solve(a: Vec<u8>) -> usize {
    a.windows(3)
        .filter(|&a| a[0] != a[2] && a[0] != a[1] && a[1] != a[2] && (a[1] > a[0]) == (a[1] > a[2]))
        .count()
}

fn output(ans: usize) -> String {
    ans.to_string() + "\n"
}