from collections import Counter

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

ans = 0
for i in range(n - 2):
    bl1 = max(Counter(a[i:i+3]).values()) == 1
    bl2 = sorted(a[i:i+3])[1] != a[i+1]
    ans += bl1 * bl2

print(ans)