I have a code for limiting input field to numeric value. Can I add "-" as an exclusion?

Question

I have this code

function isNumberKey(evt){
var charCode = (evt.which) ? evt.which : event.keyCode
if (charCode > 31 && (charCode < 48 || charCode > 57))
    return false;
return true;
}

html command

onkeypress="return isNumberKey(event)"

I want to be able to type the minus sign in the input field. Please help. Thanks in advance.