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

#[allow(dead_code)]
fn get_line() -> String {
    let stdin = stdin();
    let mut line = String::new();
    stdin.lock().read_line(&mut line).expect("io error.");
    line.trim().to_string()
}

#[allow(dead_code)]
fn get_word<T: FromStr>() -> T {
    (&get_line()).parse().ok().expect("parse error.")
}

#[allow(dead_code)]
fn get_vec<T: FromStr>() -> Vec<T> {
    (&get_line()).split(' ').map(|x| x.parse().ok().expect("parse error.")).collect()
}

fn main() {
    let n = get_word::<usize>();
    let a = get_vec::<u64>();

    let mut ans: u64 = 0;
    for i in 1..n-1 {
        let mut b = (&a[i-1..i+2]).clone().to_vec();
        // println!("{:?}", b);
        b.sort();
        if b[1] != a[i] && b[0] != b[1] && b[1] != b[2] {
            ans += 1;
        }
    }

    println!("{}", ans);
}