Problem #PRU-100527

Problem

Each pair of cities in Wonderland is connected by a flight operated by "Wonderland Airlines". How many cities are there in the country if there are $105$ different flights? We count a flight from city $A$ to city $B$ as the same as city $B$ to city $A$ - i.e. the pair $A$ to $B$ and $B$ to $A$ counts as one flight.