// Sums up all unitless values in $list
// -------------------------------------------------------------------------------
// @documentation http://sassylists.com/documentation/#sum
// -------------------------------------------------------------------------------
// @example sum(1 2 3 4 5)         => 15
// @example sum(1 a 2 b 3)         => 6
// @example sum(10px 3em 5%)       => 0
// @example sum(10px 3em 5%, true) => 18
// -------------------------------------------------------------------------------
// @param $list [List] : list
// @param $force [Boolean] : enable / disable parseInt
// -------------------------------------------------------------------------------
// @return [Number]

@function sum($list, $force: false) {
    $result: 0;

    @each $item in $list {
        @if type-of($item) == number {

            @if $force and not unitless($item) {
                $item: $item / ($item * 0 + 1);
            }

            @if unitless($item) {
                $result: $result + $item;
            }

        }
    }

    @return $result;
}