#include<iostream>
#define REP(i,a) for(i=0;i<a;i++)
using namespace std;

int kadomatsu(int a,int b,int c) {
    if(a<b && b>c && a!=c) { return 1; }
    else if(a>b && b<c && a!=c) { return 1; }
    else { return 0; }
}

int main () {
    int n;
    cin >> n;
    int a,b,c,i;
    cin >> a >> b >> c;
    int cnt =kadomatsu(a,b,c);
    REP(i,n-3) {
        a=b;
        b=c;
        cin >> c;
        cnt += kadomatsu(a,b,c);
    }
    cout << cnt << endl;
    return 0;
}