function calc_form(){
  var quant = document.getElementById('quant');
  var period_field = document.getElementById('period');
  var period_index = period_field.selectedIndex;
  var calc_result = document.getElementById('calc_result');
  var value = 1;
  var quantity = '';
  
  var weekInMonth = 30 / 7;
  var bottleInMonth = 1.2;
  var err = false;
  
  quant.value = quant.value.replace(/\D+/g, "");
  if(quant.value.search(/^[0-9]+$/)){
    err = true;
    alert('Укажите количество человек');
  }
  
  if(!err){
    if(period_index==0)
      value = bottleInMonth/weekInMonth;
    else if(period_index==1)
      value = bottleInMonth;
    else if(period_index==2)
      value = bottleInMonth * 3;
    quantity = Math.ceil(value * quant.value);
  }
  
  calc_result.innerHTML = quantity;
}