#include <bits/stdc++.h>
using namespace std;
void input(vector<long long>& v, long n) {
  for (long long i = 0; i < n; i++) {
    long long in;
    cin >> in;
    v.push_back(in);
  }
}
void output(vector<long long> v) {
  for (long long i = 0; i < v.size(); i++) cout << v[i] << " ";
}
long long max_of_three(long long a, long long b, long long c) {
  if (a >= b && a >= c)
    return a;
  else if (b >= a && b >= c)
    return b;
  else
    return c;
}
long long max_of_4(long long a, long long b, long long c, long long d) {
  if (a >= b && a >= c && a >= d)
    return a;
  else if (b >= a && b >= c && b >= d)
    return b;
  else if (c >= a && c >= d && c >= b)
    return c;
  else
    return d;
}
map<long long, long long> frequency(vector<long long> v) {
  map<long long, long long> m;
  for (long long i = 0; i < v.size(); i++) {
    if (m.find(v[i]) != m.end())
      m[v[i]]++;
    else {
      m.insert({v[i], 1});
    }
  }
  return m;
}
long long max_occuring_int(std::map<long long, long long> m) {
  long long max_x = 0;
  long long i = 0, n = m.size();
  std::map<long long, long long>::iterator it = m.begin();
  for (i = 0; i < n; i++) {
    if (it->second > max_x) max_x = it->second;
    it++;
  }
  return max_x;
}
vector<long long> give_unique_in_vector(vector<long long> v) {
  sort((v).begin(), (v).end());
  auto it = unique((v).begin(), (v).end());
  v.resize(std::distance(v.begin(), it));
  vector<long long> x = v;
  return x;
}
void solve() {
  long long n;
  cin >> n;
  vector<long long> v;
  input(v, n);
  long long ans = 0;
  for (long long i = 1; i <= n - 2; i++) {
    if ((v[i] > v[i + 1]) && (v[i] > v[i - 1]))
      ans++;
    else if ((v[i] < v[i - 1]) && (v[i] < v[i + 1]))
      ans++;
  }
  cout << ans;
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  solve();
  return 0;
}
