js多个输入框运算计算结果输出到另一个输入框

<div id="ckjrzy_4_14_15">

               <div id="ckjrzy_4_14_17">收益计算器

               </div>

               <div id="ckjrzy_4_14_30">

               <img src="images/zy/官方网改版---首页-2016_55_副本.jpg" />

               </div>

               周期:<input id="zhouqi" type="text" name="zhouqi"  />天

               投资金额: <input id="22" type="text" name="jine"  />元

               年化利率:   <input id="33" type="text" name="nianhua"  />%

              <input id="114" type="button" name="jisuan"  value="="  onclick="myFunction()"/>

              预计收益:    <input id="shouyi" type="text" name="shouyi"   value="" />

                    元

          </div>

            <input id="shouyi" type="text" name="shouyi2"   value="" />

 <script type="text/javascript">

function myFunction(){

       var zhouqi=document.getElementById("zhouqi").value;

    var jine=document.getElementById("22").value;

    var nianhua=document.getElementById("33").value;

    var sum = parseInt(nianhua)/365/100*parseInt(jine)*parseInt(zhouqi);

    document.getElementById("shouyi").value=sum;

}

</script>

<script language="javascript" type="text/javascript">

        <!--

        function f1(){

            var num1 = document.getElementsByTagName('input')[0].value;

            var num2 = document.getElementsByTagName('input')[1].value;

            var sum = parseInt(num1)+parseInt(num2);

            alert('和为:'+ sum);

        }

        //-->

  </script>

 </head>

 <body>

    <input type="text"/><br />

    <input type="text"/><br />

    <input type="button" value="求和" onclick="f1()"/>

 </body>