I want to onchage "change" just one time

so: I've been trying to make an onchange change just one time, because i want the object appear on the screen after the first change, and stay on the screen:

<select required name="Tamanho" style="margin-top: 10px;margin-bottom: 10px;font-family: bebas_kairegular;font-size: 26px;" onchange="button();">
    <option value="tamanho" selected disabled id="tamanho">Escolha um tamanho</option>
    <option value="P">P</option>
    <option value="M">M</option>
    <option value="G">G</option>
</select>

<div id="button" ><button type="button" onclick="cartClick()" style="font-size: 26px;">Comprar</button></div>

and used this javascript function to appear the div on the page

  function button() {
  var p = document.getElementById("button");
  if (p.style.display === "none") {
    p.style.display = "block";
  } else {
    p.style.display = "none
  }
 }
button();

I don't know what to do to have what i want. Help me please?

Thank You!