import strutils, sequtils

let N, A = stdin.readLine.split.map parseInt
var cnt = 0

for i in 1 ..< A.high:
  if (A[i] - A[i- 1]) * (A[i] - A[i + 1]) > 0:
    if A[i- 1] != A[i + 1]:
      cnt += 1

echo cnt