Graph Coloring · Medium problem

Take $K_5$ (five mutually linked vertices) and remove just ONE edge, say the one between vertices $3$ and $4$ ($9$ links remain, out of the $10$ in $K_5$). Unlike $K_5$ itself, this graph IS planar (it satisfies the planar edge bound $E\le3V-6$ exactly, with $9=3(5)-6$). What is its chromatic number?