// #pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")

#include <bits/stdc++.h>

using namespace std;

#define BOOST 			ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0);
#define ALL(a) 			(a).begin(), (a).end()
#define SZ(a) 			(int)(a).size()
#define FOR(a, b, c) 	for (int a = (b); a <  (c); a++)
#define FORR(a, b, c) 	for (int a = (b); a >= (c); a--)
#define TRAV(a, b) 		for (auto& a : b)
#define ELO 			exit(0)
#define SORT(a)         sort(ALL(a))
#define REV(a)          reverse(ALL(a))
#define SORTR(a)        SORT(a), REV(a)
#define PB              push_back
using ll = 				long long;
using ld = 				long double; // 'long double' is 2.2 times slower than double
using pii = 			pair<int, int>;
using pll = 			pair<ll, ll>;
using vi = 				vector<int>;
using vll = 			vector<ll>;
using vpii = 			vector<pii>;
using vpll = 			vector<pll>;

template <class T> inline bool setmin(T& a, T b) {if (a > b) return a = b, 1; return 0; }
template <class T> inline bool setmax(T& a, T b) {if (a < b) return a = b, 1; return 0; }

namespace fastio {
    template <class T> istream &operator>>(istream &os, vector<T> & vec){for (auto &u : vec)os >> u;return os;}
    template <class T> ostream &operator<<(ostream &os, const vector<T> & vec){for (auto &u : vec)os << u << " ";return os;}
    template<class T1, class T2> inline istream& operator>>(istream& os, pair<T1, T2>& p) { return os >> p.first >> p.second; }
    template<class T1, class T2> ostream& operator<<(ostream& os, const pair<T1, T2>& p) { return os << p.first << " " << p.second; }
    template <class T> ostream &operator<<(ostream &os, set<T> & con) { for (auto &u : con) os << u << " "; return os; }

    void re() {}
    template <typename T, typename... args> void re(T& x, args&... tail) { cin >> x; re(tail...); }
    void pr() {}
    template <typename T, typename... args> void pr(T x, args... tail) { cout << x << " "; pr(tail...);}
    template <typename... args> void prln(args... tail) { pr(tail...); cout << "\n";}
}
using namespace fastio;

namespace debug {
    template < typename _T > inline void _debug(const char *s, _T x) { cerr << s << " = " << x << "\n"; }
    template < typename _T, typename... args > void _debug(const char *s, _T x, args... a) { while(*s != ',') cerr << *s++; cerr << " = " << x << ','; _debug(s + 1, a...); }
    
    #if 1
        #define debug(...) _debug(#__VA_ARGS__, __VA_ARGS__)
        #define cerr cout
    #else
        #define debug(...) 1999
        #define cerr if(0) cout
    #endif
}
using namespace debug;

//////////////_________CODE_________//////////////

constexpr int MOD = 1e9 + 7; // 998244353
constexpr int INF = INT_MAX;
constexpr ll LLINF = LLONG_MAX;

int main() {
    BOOST;
    int n; re(n);
    vi a(n); re(a);
    int res = 0;
    FOR(i, 1, n - 1) {
        vi x;
        if(i > 0) x.PB(a[i - 1]);
        x.PB(a[i]);
        if(i < n - 1) x.PB(a[i + 1]);
        SORT(x);
        x.erase(unique(ALL(x)), x.end());
        if(SZ(x) >= 2) {
            if(x[1] == a[i]) {
                res++;
            }
        }
    }
    prln(res);
    ELO;
}

Measured memory_bytes: 256