Increase in number

[Demir]

If the amount is increased, the amount is increased, but when the pressure is 10, if downwards, the amount is 300, and it should be 100. How do you fix that?

$("#kol").change(function() {
    nV = $(this).val();
    oV = this.name;
    this.name = nV;
p = $(this).parent().parent();
kol = $("#kol").val();
showsum = $("#sum").val();
sum = $("#realsum").val();

if(nV > oV) {
    mul = parseInt(sum) + parseInt(showsum);
} else {
    mul = parseInt(showsum) - parseInt(sum);
}

res = p.find("#sum");
res.html(mul);
total = 0;
$("#sum").val(mul);

});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.2.2/jquery.min.js">

</script>

Количевство: <input type="number" id="kol" min="5" value="5" step="5"><br><br>

Сумма:

<input type="hidden" id="realsum" value="100">

<input type="text" id="sum" min="100" value="100">

руб

[zymir]

$('#num').change(function() {
  $('#sum').text(this.value * this.name / this.min);
});

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js">
</script>
Количество <input id='num' type='number' value='10' min='10' step='10' name='100'>

<br>

Сумма: <span id='sum'>100</span>