#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); ++i)

int main(void)
{
  int n;
  cin >> n;
  vector<int> a(n);
  rep(i, n) cin >> a[i];

  int ans = 0;
  rep(i, n - 2)
  {
    if (a[i] == a[i + 1])
      continue;
    else if (a[i] == a[i + 2])
      continue;
    else if (a[i + 1] == a[i + 2])
      continue;
    int l = min({a[i], a[i + 1], a[i + 2]});
    int r = max({a[i], a[i + 1], a[i + 2]});
    if (l == a[i + 1] || r == a[i + 1])
    {
      ans++;
    }
  }

  cout << ans << endl;
  return 0;
}