

function Main(input) {
  input = input.split("\n");
  var n = parseInt(input[0]);
  var p = input[1].split(" ").map(i => parseInt(i));
  var ans =0;
  for(var i=1;i<n-1;i++){
    if((p[i-1]<p[i]&&p[i]<p[i+1])||(p[i-1]>p[i]&&p[i]>p[i+1])){
      ans++;
    }
  }

  console.log('%s', ans);
  return ans;
}

function debug(n){
  var input = document.getElementById("input"+n).value;
  var ans=Main(input);
  var result="WA";
  if(ans==document.getElementById("output"+n).value.split("\n")[0]){
    result="AC";
  }
  document.getElementById("result"+n).innerHTML=result;
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));