$\displaystyle \forall x_1\forall x_2\dots\forall x_{n-1},\exists x_{n}{{\neg{x_{n}=x_1}}\land\neg{x_n=x_2}\land\dot s,\land{\neg{x_n=x_{n-1}}\$

i would like to know how to show form the above sentence that every model of phi_n must have at least n elements in the underlying set