N = input()
A = map(int, raw_input().split())
res = 0
for i in range(N - 2):
    a = A[i]
    b = A[i + 1]
    c = A[i + 2]
    list1 = [[a, 1], [b, 2], [c, 3]]
    sortlist1= sorted(list1, key=lambda x:x[0], reverse=True)
    if (a == b or b == c or a == c):
        continue
    if (sortlist1[1][1] != 2):
        res += 1
print res