#!/bin/sh

sum=0

for a in $*
do
	case $a in
     	[0-9]*) ;;
     	     *) echo "Arguments must be in numbers.">&s; exit1;;
        esac
	sum=`expr $sum + $a`
done

if [ $# -lt 1 ]
then
	echo "Usage: sum <number> [<number>...]"
	exit 1
fi

if [ $# -gt 0 ]
then
echo "$sum"
exit 0
fi