#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);
    
    int N; cin >> N;
    vector<int> A(N);
    rep(i,N) cin >> A[i];

    int ans = 0;
    for(int i = 1; i < N - 1; i++) {
        int ok = 1;
        ok &= A[i - 1] != A[i] && A[i] != A[i + 1] && A[i - 1] != A[i + 1];
        int a[] = {A[i - 1], A[i], A[i + 1]};
        sort(a, a + 3);
        ok &= a[1] == A[i - 1] || a[1] == A[i + 1];
        ans += ok;
    }
    cout << ans << endl;
}