this web app is a simple calculator that adds and subtracts. my problem is that the add and the subtract buttons don't work. any suggestions?

Code:
<%
        String name = (String) session.getAttribute("name");
        if (name==null) {
                name = request.getParameter("name");
                if (name!=null) session.setAttribute("name",name);
        }
    
        String sum = (String) session.getAttribute("sum");
        if (sum==null) {
                sum = "0";
                session.setAttribute("sum",sum);
        }
        int isum = Integer.parseInt(sum);
   
        String number = request.getParameter("number");
        if (number==null) number = "0";
        int inumber = Integer.parseInt(number);
        if(request.getParameter("add") != null){
        isum += inumber;
        session.setAttribute("sum",""+isum);
        }
        if(request.getParameter("subtract") != null){
        isum += inumber;
        session.setAttribute("sum",""+isum);
        }
        %>
        <html>
        <head>
   <title>Adding Machine</title>
        </head>
        <body>
                <form method='get' action='adder.jsp'>
        <%
        if (name==null) {
        %>
                        <p>
                                Name: <input type='text' name='name' id='name'>
                        </p>
                        <p>
                                <input type='submit' name='submit' id='submit' value='signin'>
                        </p>
        <%
        } else {
        %>
                        <p>Welcome, <%=name%>!</p>
                        <p>Current sum is: <%=isum%></p>
                        <p>
                                Number to add: <input type='text' name='number' id='number' value='add'>
                        </p>
                        <p>
                             <input type='submit' name='submit' id='submit' value='add'>
                        </p>
                        <p>
                              <input type='submit' name='submit' id='submit' value='subtract'>
                        </p>
        <%
        }
        %>
                </form>
        </body>
        </html>