##### The minimum number of over/underpasses that are needed to draw a graph with no edge crossings is its genus.

Because K_{5} is not planar, it cannot be drawn in the plane without any edge crossings. However, if an over/underpass is erected on the plane, it is then possible to draw K_{5} with no edge crossings. (See the figure.) The minimum number of over/underpasses that are needed to draw a graph with no edge crossings is its genus. Thus, planar graphs have genus 0 and K_{5} has genus 1. The genus for K_{6} is _________.