Q$5$: Write a function in python that prints the least integer that is not present in a given list

and cannot be represented by the summation of the sub$-$elements of the list.

E$.$g$.$ For a $=[1,2,5,7]$ the least integer not represented by the list or a slice of the list is $4,$

and if a $=[1,2,2,5,7]$ then the least non$-$representable integer is $18.$