#include <iostream>

using namespace std;
const int N = 1010;
int q[N];
int n;

bool isCheck(int u,int v){
    if(u * v < 0) return false;
    return true;
}

int main(){
    cin >> n;
    for(int i = 1;i <= n;i++) cin >> q[i];

    if(n == 2){
        cout << 0 << endl;
        return 0;
    }

    int t = q[2] - q[1],res = 0;
    for(int i = 3;i <= n;i++){
        int u = q[i] - q[i - 1];
        if(!isCheck(t,u)) res++;
        t = u;
    }
    cout << res << endl;
    return 0;
}