// yukicoder: No.116 門松列(1)
// 2019.4.12 bal4u

#include <stdio.h>

int a[102], N;

int main()
{
	int i, ans;
	scanf("%d", &N);
	for (i = 0; i < N; i++) scanf("%d", a + i);
	ans = 0;
	for (i = 1; i < N - 1; i++) {
		if ((a[i-1] < a[i] && a[i] > a[i+1] && a[i-1] != a[i+1]) ||
			(a[i-1] > a[i] && a[i] < a[i+1] && a[i-1] != a[i+1]))
			ans++;
	}
	printf("%d\n", ans);
	return 0;
}