# How does Atiyah-Singer index theorem relates instanton number to number of fermion zero modes?

I was studying this paper, where the authors consider an $$SU(2)$$ gauge field of instanton number 1 on a 4-sphere $$M =S^4$$. If $$n_L$$ is the number of zero modes of $$\psi_L$$ and $$n_R$$ is the number of zero modes of $$\psi_R$$, then the authors mention that by using Atiyah-Singer theorem,

$$n_L - n_R = 1.$$

Can somebody elaborate why this is so?