function increaseProductsValue(fieldName) {
  var productsCountObject = document.getElementsByName(fieldName)[0];
  var productsCountStr = productsCountObject.value;

  var productsCount = parseInt(productsCountStr);

  if (!productsCount) {
    productsCount = 0;
  }

  productsCountObject.value = productsCount + 1;
}

function decreaseProductsValue(fieldName) {
  var productsCountObject = document.getElementsByName(fieldName)[0];
  var productsCountStr = productsCountObject.value;

  var productsCount = parseInt(productsCountStr);

  if (!productsCount) {
    productsCount = 1;
  }

  if (productsCount <= 1) {
    productsCountObject.value = 1;
  } else {
    productsCountObject.value = productsCount - 1;
  }
}