N = int(input())
As = list(map(int, input().split()))

def solve(N, As):
    count = 0
    for a, b, c in zip(As[:], As[1:], As[2:]):
        count += is_kadomatsu(a, b, c)
    return count

def is_kadomatsu(a, b, c):
    if a == b or b == c or a == c:
        return False
    return max(a, b, c) == b or min(a, b, c) == b

print(solve(N, As))