Nibbles, 3 bytes (6 nibbles)

<1-,~

<1      # get the first element of
   ,~   # infinite list of positive integers
  -     # with the input list subtracted from it

Nibbles, 3 2 bytes (4 nibbles)

/-,~

I somehow didn't expect that fold-ing over an infinite list, starting from infinity (the right-hand end) would work, but it does.

/-,~    # full function
/-,~$$  # (with implicit arguments shown):
 -      # remove
    $   #   elements of the input list 
        # from
  ,~    #   the infinite list of positive integers
/       # now fold over this infinite list from the right
     $  # each time returning the left-hand argument
  -     # (so, finally, returning the first (left-most) element)