Genus formula for a modular curve

Let $\mu =[PS{L}_2(\mathbb{Z}):\overline{\Gamma }]$, let $e_2,e_3$ count elliptic orbits of orders two and three, and let $c$ be the number of cusps. Then

The modular curve $X_0(3)$ has index four, two cusps, no elliptic orbit of order two, and one elliptic orbit of order three. Its genus is therefore zero.