#include<iostream>
#include<vector>
#include<algorithm>
#include<cstdio>
using namespace std;
int main(){
    vector<int> vc;
    int N,x,count=0;
    cin >> N;
    while(N--){
        cin >> x;
        vc.push_back(x);
    }
    for(int i=1;i<vc.size()-1;i++){
        if((vc[i-1]<vc[i]&&vc[i+1]<vc[i]&&vc[i-1]!=vc[i+1])||(vc[i-1]>vc[i]&&vc[i+1]>vc[i]&&vc[i-1]!=vc[i+1])) count++;
    }
    cout << count;
    return 0;
}