PL-OM 1987 Problem 1

There are $n \ge 2$ points in a square side $1$ . Show that one can label the points $P_1, P_2, ... , P_n$ such that $\sum_{i=1}^n |P_{i-1} - P_i|^2 \le 4$ , where we use cyclic subscripts, so that $P_0$ means $P_n$ .