#include <bits/stdc++.h>
using namespace std;

int main() {
int n;
cin>>n;
int A[n];
int B[n];
for (int i = 0; i < n; ++i) {
	cin>>A[i];
}
int counter=0;
for (int i = 0; i < n-2; ++i) {
	B[0]=A[i];
	B[1]=A[i+1];
	B[2]=A[i+2];
	sort(B,B+3);
	if(B[1]==A[i]||B[1]==A[i+2]){
		if(B[0]!=B[1]&&B[1]!=B[2]&&B[0]!=B[2])
		counter++;
	}
}
cout<<counter<<endl;


}