n = int(input())
A = list(map(int, input().split()))

cnt = 0
for i in range(n - 2):
    a, b, c = A[i : i + 3]
    B = sorted(A[i : i + 3])
    if a != b and b != c and c != a and (a == B[1] or c == B[1]):
        cnt += 1

print(cnt)