use std::io;

fn main() {
  let mut buf = String::new();
  io::stdin().read_line(&mut buf).ok();

  let mut buf = String::new();
  io::stdin().read_line(&mut buf).ok();
  let a: Vec<u16> = buf
    .trim()
    .split_whitespace()
    .map(|e| e.parse().ok().unwrap())
    .collect();

  let mut result = 0;

  for v in a.windows(3) {
    result += match v {
      [a, b, c] if a > c => match (a, b, c) {
        (a, b, c) if b > a || b < c => 1,
        _ => 0,
      },
      [a, b, c] if a < c => match (a, b, c) {
        (a, b, c) if b > c || b < a => 1,
        _ => 0,
      },
      _ => 0,
    };
  }
  println!("{}", result);
}