#! ruby
# yukicoder My Practice
# author: Leonardone @ NEETSDKASU

N = gets.to_i
A = gets.chomp.split.map(&:to_i)

p A.zip(A.rotate(1), A.rotate(2))[0..-3].count{|x,y,z| x!=z&&(y>[x,z].max||y<[x,z].min)}