RO-TST 1991 Problem 7

Let $x_1,x_2,...,x_{2n}$ be positive real numbers with the sum $1$ . Prove that $$x_1^2x_2^2...x_n^2+x_2^2x_3^2...x_{n+1}^2+...+x_{2n}^2x_1^2...x_{n-1}^2 <\frac{1}{n^{2n}}$$