#include <iostream>
#include <vector>
using namespace std;
int main()
{
int N;
cin>> N;
vector<int> a(N);
for(int i = 0; i < N;i++){
cin >> a[i];
}
int num = 0;
for(int i= 0; i < N-2; i++){
if(a[i] < a[i+1] and a[i+1] > a[i+2] and a[i]!=a[i+2]){
num++;
} else if(a[i] > a[i+1] and a[i+1] < a[i+2] and a[i]!=a[i+2]){
num++;
}
}
cout <<num<<endl;
return 0;
}