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

int N, A[100], B[3];

int main(){
    cin >> N;
    REP(i,N){
        cin >> A[i];
    }
    int res = 0;
    REP(i,N-1){
        if(i == 0) continue;
        if(A[i-1] == A[i] || A[i] == A[i+1] || A[i-1] == A[i+1]) continue;
        B[0] = A[i-1], B[1] = A[i], B[2] = A[i+1];
        sort(B,B+3);
        if(A[i-1] == B[1] || A[i+1] == B[1]) res++;
    }

    cout << res << endl;
}