Given a number oct$\_$num in octal system $($base $8),$ return oct$\_$num in decimal system $($base $10).$ In

the octal number system, each digit represents a power of eight and it uses the digits $1$ to $7.$ To

convert a number represented in octal system to a number represented in decimal system, each

digit must be multiplied by the appropriate power of eight. For example, given the octal number

$2068$ results in the decimal number $13410$