lines = []
while line = gets
    lines << line.chomp.split(' ').map(&:to_i)
end

N = lines[0][0]

ans = 0
for i in 2..N-1 do
  if lines[1][i-2..i].sort[1] == lines[1][i-1] then
    ans += 1
  end
end
  
print ans