Three cities are connected such that each city has atleast one direct route to each of other.If one wants to go from A to B,either a direct route (AB) or an indirect route (ACB) can be used.Totally,there are 33 routes from A to B and 23 routes from B to C.How many indirect routes are there from A to C?

1) 18
2) 15
3) 16
4) 21