use std::io;

fn main() {
  let mut buf = String::new();
  io::stdin().read_line(&mut buf).ok();
  let n: usize = buf.trim().parse().ok().unwrap();

  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 i in 0..n - 2 {
    
    let v1 = a[i];
    let v2 = a[i + 1];
    let v3 = a[i + 2];

    if v1 != v3 && v1 != v2 && v2 != v3 {
      let (mut max, mut min) = if v1 < v3 { (v3, v1) } else { (v1, v3) };
      if v2 < min || v2 > max {
        result += 1;
      }
    }
  }
  println!("{}", result);
}