Isomorphism

Prove that if two graphs $A$ and $B$ do not have the same number of nodes, they cannot be isomorphic. I have started with the formal definition of isomorphism below. Add your answer to this markdown file. This page might help with the notation for mathematical expressions.

$G_1=(V_1 , E_1)$ is isomorphic to $G_2 = (V_2, E_2)$ if there exists a one-to-one and onto function (bijection) $f: V_1 \rightarrow V_2$ such that $(u,v) \in E_1$ iff $(f(u),f(v)) \in E_2$.