[yashka525]

<div class="cena" id="cena">199</div>
    <div><input type="text" value="1" class="num" id="num" onblur="Add()"></div>
    <div class="sum" id="sum">199</div>
<script>
function Add(){
    var Cena = document.getElementById('cena');
    var Num = document.getElementById('num');
    var Sum = document.getElementById('sum');
    Sum.innerHTML = Cena.innerHTML * Num.value;
}
</script>