n = gets.chomp.to_i
m = gets.chomp.split.map(&:to_i)
c = 0
(n - 2).times do | i |
if m[i] != m[i+2]
if m[i] > m[i+1] && m[i+1] < m[i+2]
c += 1
elsif m[i] < m[i+1] && m[i+1] > m[i+2]
c += 1
end
end
end
puts c