let i = require('fs').readFileSync('/dev/stdin', 'utf8').split('\n')
let n = parseInt(i[0])
let a = i[1].split(' ').map((e) => parseInt(e))
let c = 0
for (let j = 0; j < a.length - 2; j++) {
  let t = a.slice(j, j+3)
  if ((t[0] - t[1]) * (t[2] - t[1]) > 0 && t[0] != t[2]) c++
}
console.log(c)