What's 15?

If $1\implies 2468$

$2\implies 15$

$3\implies 76857$

$4\implies -897$

$5\implies 13546$

$6\implies -8$

$7\implies 0$

$8\implies 89$

$9\implies 864$

$10\implies 554$

$11\implies 78$

$12\implies 888888$

$13\implies 216$

$14\implies 196$

$15\implies ?$