function compute(){
		var f = self.document.forms['sales'];
		w = f.weight.value;

		if (!chekw(w)){
			alert("number please!");
			f.weight.focus();
			return;
		}
	
		if(w<=0) s = 'Fehler';
		else {
			if(w<1000) {
				if(w<=50)s = 6;
				else s = Math.ceil(w/100)*3+6;
			}
			else s = Math.ceil((w-1000)/250)*3+36;
		}
	
		if(w<=0) m = 'Fehler';
		else {
			if(w<1000) {
				if(w<=50)m = 12;
				else m = Math.ceil(w/100)*6+12;
			}
			else m = Math.ceil((w-1000)/250)*12+72;
		}
	
		if(w<=0) l = 'Fehler';
		else {
			if(w<1000) {
				if(w<=50)l = 18;
				else l = Math.ceil(w/100)*9+18;
			}
			else l = Math.ceil((w-1000)/250)*18+108;
		}

		f.small.value = s;
		f.medium.value = m;
		f.large.value = l;
	}

	function chekw(w){
		if (isNaN(w)){
			return false;
		}
		else{
			return true;
		}
	}