#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define srep(i,s,t) for (int i = s; i < t; ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
#define yn {puts("Yes");}else{puts("No");}
#define MAX_N 200005

int main() {
    int n;
    cin >> n;
    int a[n];
    rep(i,n)cin >> a[i];

    int ans = 0;

    rep(i,n-2){
        int x = a[i], y = a[i+1], z = a[i+2];
        if(x != y && y != z && z != x){
            if(y < x && y < z)ans++;
            if(y > x && y > z)ans++;
        }
    }

    cout << ans << endl;
    return 0;
}