#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  int a[n];
  for (int i = 0; i < n; i++) {
    scanf("%d", a + i);
  }
  int countmin = 0, countmax = 0;
  for (int i = 1; i < n - 1; i++) {
    if (a[i - 1] > a[i] && a[i + 1] > a[i]) countmin++;
    if (a[i - 1] < a[i] && a[i + 1] < a[i]) countmax++;
  }
  cout << countmin + countmax;
  return 0;
}
