Update field with sum in form html/php

I need help !

I have a form done in html , with two fields ( value a and b value ) and a field that will be updated with the sum of the values ​​placed in the fields mentioned above (a and b ) . It turns out that already researched , and I can not get that to work.

The form is as follows:

        <form method="post" action="confirmar_mov.php"> 

            <label for="data">Data do Documento</label> &nbsp;&nbsp;&nbsp;
            <input id="data" name="data" placeholder="dd-mm-yyy" required="" type="date">
            &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
            <br><br>
            <label for="conta">Conta associada (SNC) </label> &nbsp;&nbsp;&nbsp;
            <select name="conta">
                <option value="11">Caixa</option>
                <option value="2111">Clientes</option>
                <option value="2211">Fornecedor</option>
            </select>

            <br><br>
            <div>
                <div style="float:left;margin-left:10%;">
                    <h2> ENTRADA </h2>
                    <label for="valor_entrada">Valor:</label>&nbsp;&nbsp;&nbsp;
                    <input id="valor_entrada" name = "valor_entrada" type="text" > 
                </div>
                <div style="float:right;margin-right:40%;">
                    <h2> SAÍDA </h2>
                    <label for="valor_saida">Valor:</label>&nbsp;&nbsp;&nbsp;
                    <input id="valor_saida" name = "valor_saida" type="text"> 
                </div>

            </div>

            <br><br><br><br><br><br><br><br>

            <table style="width:100%">

              <tr>
                <td><b>Total do Movimento</b></td>
                <td><input id="sum" name="sum" type="text"></td>        
              </tr>
            </table>
            <br><br>

            <input class="button" name="submit" id="submit" value="Registar" type="submit">      

Basically , the value corresponds to "Entrada" and b corresponds to " Saída" and the sum is " Total" .

Someone help me? it's urgent