#include <bits/stdc++.h>
#define rep(i,n) for(ll (i)=0;(i)<(n);(i)++)
#define repi(i,a,b) for(ll (i)=(a);(i)<(b);(i)++)
#define SIZE_OF_ARRAY(array)    (sizeof(array)/sizeof(array[0]))
typedef long long ll;
using namespace std;
int main(){
  int n;
  cin >> n;
  int a[n];
  int ans = 0;
  rep(i,n) cin >> a[i];
  for(int i=0;i<n-2;i++){
    if(a[i]!=a[i+2]){
    if((a[i+1]>a[i]) && (a[i+1]>a[i+2])){
      ans++;
      continue;
    }
    if((a[i+1]<a[i]) && (a[i+1]<a[i+2])){
      ans++;
    }
    }
  }
  cout << ans << endl;
  return 0;
  
}