#include <bits/stdc++.h>
#define int long long
#define double long double
using namespace std;
const int MOD = 1000000007;
const int INF = 1e14;
using Graph = vector<vector<int>>;

signed main(){
  int N;
  cin >> N;
  vector<int> A(N);
  for( int i = 0; i < N; i++ ) cin >> A[i];

  int ans = 0;
  for( int i = 0; i < N-2; i++ ){
    if( A[i] == A[i+1] || A[i+1] == A[i+2] || A[i+2] == A[i] ) continue;
    vector<int> B(3);
    B[0] = A[i]; B[1] = A[i+1]; B[2] = A[i+2];
    sort(B.begin(), B.end());
    if( A[i] == B[1] || A[i+2] == B[1] ) ans++;
  }
  cout << ans << endl;
}