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


def kado(a, b, c):
    if len(set([a, b, c])) != 3:
        return False
    m = sorted([a, b, c])[1]
    if a == m or c == m:
        return True
    return False


ans = 0
for i in range(N - 2):
    ans += int(kado(*A[i:i+3]))

print(ans)