function calculate_total() {
  var total = 0
  items = document.getElementsByClassName("order_item")
  for (const item of items){
    price = parseInt(item.dataset.price)
    count = parseFloat(item.value)
    total += count * price
  }
  document.getElementById("total_price").innerText = total + "zł"
}

function min(a, b) {
  if (a > b) {
    return a
  }
  return b
}

function increase(id) {
  input = document.getElementById(id)
  step = parseFloat(input.step)
  input.value= min(parseFloat(input.value)+step, 0)
  calculate_total()
}

function decrease(id) {
  input = document.getElementById(id)
  step = parseFloat(input.step)
  input.value= min(parseFloat(input.value)-step, 0)
  calculate_total()
}

calculate_total()