#!/bin/sh

usage () {
	echo usage: `basename $0` \<positive number\> && exit 1
}

calc() {
[ $1 -lt 0 ] && usage
[ $1 -lt 2 ] && echo 1 && return

n=1;jog=2
while :;do
	n=`expr $n \* $jog`
	jog=`expr $jog + 1`
	[ $jog -gt $1 ] && break;
done

echo $n && return
}

for i in 1 2 3 4 5 6 7 8 9 10 11	\
	12 13 14 -1; do
	echo "$i! =" `calc $i`
done

exit 0