#include <iostream>

#define FORR(i,b,e) for(int i=(b);i<(int)(e);++i)
#define FOR(i,e) FORR(i,0,e)
#define dump(var) cerr << #var ": " << var << "\n"
#define dumpc(con) for(auto& e: con) cerr << e << " "; cerr<<"\n"

typedef long long ll;
typedef unsigned long long ull;

const double EPS = 1e-6;
const int d4[] = {0, -1, 0, 1, 0};

using namespace std;


int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    int N, a[110];
    cin >> N;
    cin >> a[0] >> a[1];
    int d1 = a[1]-a[0], d2, cnt = 0;
    FORR(i, 2, N) {
        cin >> a[i];
        d2 = a[i]-a[i-1];
        cnt += (d1*d2 < 0 && a[i]!=a[i-2]);
        d1 = d2;
    }
    cout << cnt << endl;
    
    return 0;
}