#include <bits/stdc++.h>
using namespace std;
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long n;
  cin >> n;
  long long a[n];
  for (long long i = 0; i < n; ++i) cin >> a[i];
  long long ans = 0;
  for (long long i = 1; i < n - 1; ++i) {
    if (a[i] < a[i - 1] && a[i] < a[i + 1])
      ++ans;
    else if (a[i] > a[i - 1] && a[i] > a[i + 1])
      ++ans;
  }
  cout << ans;
}
