#include <bits/stdc++.h>

using namespace std;

#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define REP(i,n) FOR(i,0,n)
#define ALL(a) (a).begin(),(a).end()

int main(){
    int n;cin>>n;
    vector<int> a(n);
    REP(i,n){
        cin>>a[i];
    }
    int res=0;
    int sub[3]={a[0],a[1],a[2]};
    cerr<<sub[0]<<"\t"<<sub[1]<<"\t"<<sub[2]<<endl;
    if((sub[0]!=sub[2]) and ((sub[0]<sub[1] and sub[1]>sub[2]) or (sub[0]>sub[1] and sub[1]<sub[2]))){
        cerr<<"Detected"<<endl;
        res++;
    }
    REP(i,n-3){
        sub[0]=sub[1];
        sub[1]=sub[2];
        sub[2]=a[3+i];
        cerr<<sub[0]<<"\t"<<sub[1]<<"\t"<<sub[2]<<endl;
        if((sub[0]!=sub[2]) and ((sub[0]<sub[1] and sub[1]>sub[2]) or (sub[0]>sub[1] and sub[1]<sub[2]))){
            res++;
            cerr<<"Detected"<<endl;
        }
    }
    cout<<res<<"\n";
    return 0;
}