//HTML

<label for="test1">Value(€)</label>
<input type="text" name="test1" id="test1" value="0"><br>



//JAVASCRIPT FUNCTION


function selected_feature(event){
    //set to 0
        document.getElementById('test1').value = 0;

    //Loop
    for(var i=0; i<elements.selectedFeatures.length; i++){

        var elements;
        elements += parseFloat(elements.selectedFeatures[i].attributes.value_elements);
        document.getElementById('test1').value = elements;
        }

    }