function checkBox() {
  let ul = document.getElementsByTagName('ul')[0];
  let list =  document.getElementsByTagName('li');
  let numOfChecked = 0;
  let checkedBoxes = document.querySelectorAll("input[name=checkbox]");

  if (checkedBoxes.length === 0) {
  complete.innerHTML = numOfChecked + '/' + checkedBoxes.length;
  }

  for (let i =0; i<checkedBoxes.length; i++){
    if (checkedBoxes[i].checked === true) {
        list[i].style.textDecoration = "line-through";
        list[i].style.backgroundColor = '#888';
        list[i].style.color = '#fff';
        numOfChecked++;
        complete.innerHTML = numOfChecked + '/' + checkedBoxes.length;
      }


      else if (checkedBoxes[i].checked === false){
        list[i].style.textDecoration = "none";
        numOfChecked.value = checkedBoxes.length;
        complete.innerHTML = numOfChecked + '/' + checkedBoxes.length;
      }
}}