import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class No116 {
	public static void main(String[] args) throws IOException{
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		int n = Integer.parseInt(br.readLine());
		String[] str = br.readLine().split(" ");
		int[] num = new int[n];
		for(int i=0; i < n; i++) num[i] = Integer.parseInt(str[i]);
		System.out.println(judge(num));
	}
	
	public static int judge(int[] a){
		int ans = 0;
		for(int i=1; i < a.length - 1; i++){
			if(a[i-1] != a[i] && a[i-1] != a[i+1] && a[i] != a[i+1]){
				if(!(a[i-1] > a[i] && a[i] > a[i+1]) && !(a[i-1] < a[i] && a[i] < a[i+1])){
					ans++;
				}
			}
		}
		return ans;
	}
}