Transformation of a Lagrangian

Question

Starting from the Lagrangian \(L=-\partial_{\mu}\phi^{a}\partial^{\mu}\phi^{a}/2-\mu^{2}\phi^{a}\phi^{a}/2-\lambda(\phi^{a}\phi^{a})^{2}/4!\)and taking \(\mu^{2} \rightarrow -\infty, \lambda \rightarrow \infty\)such that \(<\phi^{a}\phi^{a}>\)is finite and droping any infinite mass fields, I want to get the Lagrangian \[L=-f^{2}\partial_{\mu}\sigma^{a}\partial^{\mu}\sigma^{a}/2\]where a=1,2,3,4, f a constant with units of mass and we also have the constraint \(\Sigma \sigma^{a}\sigma^{a}=1\). Could you give me any hint on how I should start?