import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

N,*A = map(int,read().split())

answer = 0
for x,y,z in zip(A,A[1:],A[2:]):
    answer += ((x < y > z) or (x > y < z)) and (x != z)

print(answer)