Since n - sn equals the number of tails in n tosses it

Question

Toss a symmetric coin repeatedly and set

Since n - S_n equals the number of tails in n tosses, it follows that X_n = the number of heads minus the number of tails, that is the excess of heads over tails in n tosses. Show that {X_n, n ≥ 1} (together with the sequence of natural σ-algebras) is a martingale.