input()
A = list(map(int, input().split()))
ans = 0
for i in range(len(A)-2):
	S = A[i:i+3]
	S.sort()
	if(S[1] == A[i] or S[1] == A[i+2]):
		if(A[i] != A[i+1] and A[i+1] != A[i+2] and A[i] != A[i+2]):
			ans += 1
print(ans)