n = int(input())
seq = input().split()
count = 0
for i in range(n - 2):
    if seq[i] == seq[i+2]:
        continue
    elif int(seq[i]) > int(seq[i+1]) and int(seq[i+1]) < int(seq[i+2]):
        count += 1
    elif int(seq[i]) < int(seq[i+1]) and int(seq[i+1]) > int(seq[i+2]):
        count += 1
print(count)