N = int(input())
A = input().split()
A = [int(i) for i in A]


def ch(int1, int2, int3):
    if (int1 == int2) or (int2 == int3) or (int1 == int3):
        return 0
    elif ((int1 < int2) and (int1 > int3)) or ((int1 > int2) and (int1 < int3)):
        return 1
    elif ((int3 < int1) and (int3 > int2)) or ((int3 > int1) and (int3 < int2)):
        return 1
    else:
        return 0
ch

count = 0
for i in range(0, N-2):
    count = count + ch(A[i], A[i+1], A[i+2])

print(count)