JSP :: Increment Variable From JavaScript - Function Is Undefined

I've a simple logic Code to increment a variable from javascript and Send it back to jsp. I just writing simple code to explain the logic. My code will have scritplets. I know usage of scriptlets is inside jsp is oldest but for the sake of logic.

<!--Jsp Page -->
<%
int i=0;
<input type="button" onclick="fun()"/>
%>
<!-- Script as Follows-->
<script>
function fun(){
<%i++;5>
}
</script>

But My value of i is not incrementing its showing that function is undefined