#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); i++)
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<ll> vll;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef vector<vector<ll>> vvll;
const ll inf = 1e16;
const ll md = 1000000007;

int aa[105];

int main() {
  int n;
  cin >> n;
  rep(i, n) cin >> aa[i];
  int ans = 0;
  rep(i, n - 2) if ((aa[i + 1] - aa[i]) * (aa[i + 1] - aa[i + 2]) > 0 &&
                    aa[i] != aa[i + 2]) ans++;
  cout<<ans<<endl;
  return 0;
}