Get Minimum Round Trip Cost (: for E4 && E5 :)

There are 2 arrays which denote departing and returning flights with the respective indexes being time and the values of the array being the cost it takes for the flight. Return the minimum cost for a round trip provided the return flight can only be taken at a time post departing flight time. (i.e if departing at time i, one can catch a returning flight only from time (i+1) onwards). For example, departing = [1,2,3,4] and returning = [4,3,2,1], the minimum cost for round trip will be 2 i.e departing[0] + returning[3]. Solve this in O(n) time.