graph algorithm question

hi, all
Suppose I have cities and have airlines connect some of them (bi-
directional, labeled with price). I want to find a "round trip" such
that the average of flight cost is minimum. The "round trip" is
defined as following: it must contains at least 3 cities, each passes
only once except the starting city which will be the ending city as
well.
Is there already an algorithm to deal with this problem? Is this an NP-
hard problem? Thanks.
zl2k
.