Self-intersection of exceptional sphere

Question

Let $M$ be a smooth close $4-$dimensional manifold and let $p\in M$ be a point. Denote by $\hat{M}$ the blow-up of $M$ at $p$ and let $E \cong \mathbb{C}P^{1}$ denote the exceptional sphere which is embedded. How can one show that the self-intersection number of $E$ is $-1$, i.e. $E \cdot E = -1$?