Let $F$ be a finite field extension of the $p$-adic numbers $\mathbb{Q}_p$, whose residue field has $q$ elements. Let $\mathfrak{p}$ be the prime ideal of $F$. Given a finite field extension $K/F$, write $\operatorname{Disc}_\mathfrak{p}(K)$ for $q^{v_\mathfrak{p}(\operatorname{Disc}(K/F))}$. Then **Serre's mass formula** states that for all $n\geq 1$, we have

$$ \sum_{[K:F] = n \text{ totally ramified}} \frac{1}{\#\operatorname{Aut}(K/F)}\cdot\frac{1}{\operatorname{\operatorname{Disc}_{\mathfrak{p}}(K)}} = \frac{1}{q^{n-1}}. $$

This result first appeared in Serre's 1978 paper **Une “ formule de masse ” pour les extensions totalement ramifiees de degre donne d’un corps local**. As far as I can tell, this paper is only available in the book **Oeuvres/Collected Papers. 3 volumes.** Given that I am unable to obtain this book, I am looking for a reference on the internet that contains either Serre's original paper or an exposition of his proof (ideally both proofs from the paper).

I would also (delightedly) accept an answer that directly explains the proof (i.e. no reference); such an answer would in my opinion be a great addition to the learning resource that is MO, seeing as the proof is so hard to find on the internet!