如何才能使“复选框”在每次左击时增加其数量，在右击时减少？

// HTML代码
<input type="checkbox" id="checkbox" />
<span id="count">0</span>

// JavaScript代码
var checkbox = document.getElementById("checkbox");
var countSpan = document.getElementById("count");
var count = 0;

checkbox.addEventListener("click", function() {
  count++;
  countSpan.textContent = count.toString();
});

checkbox.addEventListener("contextmenu", function(e) {
  e.preventDefault(); // 阻止默认的右击菜单弹出
  count--;
  countSpan.textContent = count.toString();
});