#include <iostream>
#include <iomanip>
#include <vector>

typedef long long ll;

int main(void)
{
  std::cin.tie(0);
  std::ios::sync_with_stdio(false);
  std::cout << std::fixed << std::setprecision(8);
  int n;
  std::cin >> n;
  int a[n];
  for (int i = 0; i < n; ++i) std::cin >> a[i];
  int ans = 0;
  for (int i = 1; i < n-1; ++i) {
    if ((a[i] < a[i-1] && a[i-1] < a[i+1]) ||
        (a[i+1] < a[i-1] && a[i-1] < a[i]) ||
        (a[i] < a[i+1] && a[i+1] < a[i-1]) ||
        (a[i-1] < a[i+1] && a[i+1] < a[i])) ans++;
  }
  std::cout << ans << std::endl;
  return 0;
}