Restrict User to Enter Value up to 2 Decimal Places in JavaScript

Html Input:

<input id="Input" name="Quantity1" type="text" onkeypress="return IsDigit(event,this.id)" />

JavaScript:

<script type="text/javascript">
function IsDigit(evt, elementId)
{
    var charCode = evt.charCode;
    if (charCode > 31 && (charCode < 48 || charCode > 57) && charCode != 46 && charCode != 0)
    {
        return false;
    }
    else
    {
        var len = document.getElementById(elementId).value.length;
        var index = document.getElementById(elementId).value.indexOf('.');
        if (charCode == 0)
        {
            return true;
        }
        if (index > 0 && charCode == 46)
        {
            return false;
        }
        if (index > 0)
        {
            var CharAfterdot = (len + 1) - index;
            if (CharAfterdot > 3)
            {
                return false;
            }
        }
    }
    return true;
}
</script>