import java.util.Arrays;
import java.util.Scanner;
public class No116 {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		// N本の竹を配列に格納
		int[] take = new int[n];
		for(int i = 0; i < n; i++){
			take[i] = sc.nextInt();
		}
		sc.close();
		int kadomatuCount = 0;
		for(int i = 0; i < n - 2; i++){
			// いずれかが同じ長さであれば回避
			if(take[i] == take[i + 1] || take[i + 1] == take[i + 2] || take[i] == take[i + 2]){
				continue;
			}
			// 隣接する3本を1つの別の配列にする
			int[] takeTemp = new int[3];
			takeTemp[0] = take[i];
			takeTemp[1] = take[i + 1];
			takeTemp[2] = take[i + 2];
			// 3本を並べ替え
			Arrays.sort(takeTemp);
			// 2番目が左端か右端なら門松列
			if(takeTemp[1] == take[i] || takeTemp[1] == take[i + 2]){
				kadomatuCount++;
			}
		}
		System.out.println(kadomatuCount);
	}
}