how to cal javascript method in server side button control?

function output() {

       var tamount = parseFloat(document.getElementById('<%=lblopeningbirds.ClientID%>').innerHTML);
       var cash = parseFloat(document.getElementById('<%=txtmort.ClientID%>').value);
       if (isNaN(tamount) != true && isNaN(cash) != true) {
           document.getElementById('<%=lblclosingbirds.ClientID%>').innerHTML =
           Math.round(
           parseFloat(document.getElementById('<%=lblopeningbirds.ClientID%>').innerHTML) -
           parseFloat(document.getElementById('<%=txtmort.ClientID%>').value)

           );

           return false;
       }

   }