#include<bits/stdc++.h>
#define rep(i,a,n) for (int i = a;i < n;i++)
#define per(i,n,a) for (int i = n-1;i >= a;i--)
using namespace std;

int check(int a, int b, int c){
    if (a == b || a == c || b == c) return 0;
    if (max(a, max(b, c)) + min(a, min(b, c)) != a + c) return 1;
    return 0;
}

int main() {
    int N;
    cin >> N;
    vector<int> A(N);
    rep(i,0,N){
        cin >> A[i];
    }

    int ans = 0;
    rep(i,0,N-2){
        ans += check(A[i], A[i + 1], A[i + 2]);
    }
    cout << ans << endl;
}