Javascript/平均値

var count = 0;
var sumh = 0;
var sumw = 0;
var nam = document.getElementById("nam");
var stature = document.getElementById("stature");
var weight = document.getElementById("weight");
var list = document.getElementById("list");
var average = document.getElementById("average");
var btn = document.getElementById("btn");
btn.disabled = true;

function num(){
  if(!this.value.match(/^[0-9]+$/)){
  	alert("数字のみ");
  	this.value = "";
  }
  if(nam.value != "" && stature.value != "" && weight.value != ""){
  	btn.disabled = false;
  }
}
nam.addEventListener("change", function(){
  if(nam.value != "" && stature.value != "" && weight.value != ""){
  	btn.disabled = false;
  }
});
stature.addEventListener("change", num);
weight.addEventListener("change", num);

btn.onclick = function sFunc(){
  count++;
  sumh += parseFloat(stature.value);
  sumw += parseFloat(weight.value);
  var tr = document.createElement("tr");
  var td01 = document.createElement("td");
  td01.appendChild(document.createTextNode(nam.value));
  var td02 = document.createElement("td");
  td02.appendChild(document.createTextNode(stature.value + "cm"));
  var td03 = document.createElement("td");
  td03.appendChild(document.createTextNode(weight.value + "kg"));
  tr.appendChild(td01);
  tr.appendChild(td02);
  tr.appendChild(td03);
  list.appendChild(tr);
  var averageH = sumh / count;
  averageH = averageH.toFixed(1);
  var averageW = sumw / count;
  averageW = averageW.toFixed(1);
  average.innerHTML = "p>平均身長" + averageH + "cm/p>" 
  						+ "p>平均体重" + averageW + "kg/p>";
  nam.value = "";
  stature.value = "";
  weight.value = "";
}