function isNumber(obj,danjia){
 isNum = /^[0-9]*$/;
 if(!isNum.test(obj.value))
   obj.value=1;
   sun=obj.value*danjia
   document.getElementById("jisuan").innerHTML="$"+danjia.toFixed(2)+" × "+obj.value+" = $"+sun.toFixed(2)+"";
}