JSFiddle - React, Tailwind, and code Playground

HTML

<form action="#" method="post" id="Form" name="Form">    
<p><label title="Veuillez saisir la distance parcourue" for="dist">distance :</label>
<input type="text" size='6' maxlength='6' title="Veuillez saisir la distance parcourue" id="dist" name="dist" value="2" onkeyup="updateResult()" /> km
</p>
<p><label title="Veuillez saisir la distance parcourue" for="dist2">distance 2 :</label>
<input type="text" size='6' maxlength='6' title="Veuillez saisir la distance parcourue" id="dist2" name="dist2" value="5" onkeyup="updateResult()" /> km
<br/>
</p>
<p>Total km:
<input type="text" size='20' maxlength='6' title="Total Km D&eacute;placement" id="totalKm" name="TotalKm" readonly />
</p>
</form>

JavaScript

function updateResult() {
    var dist=parseInt(document.getElementById('dist').value); 
    var dist2=parseInt(document.getElementById('dist2').value);
    document.getElementById('totalKm').value=dist+dist2;
}