CMO 1975 Problem 3

For each real number $r$ , $[r]$ denotes the largest integer less than or equal to $r$ , e.g. $[6] = 6, [\pi] = 3, [-1.5] = -2$ . Indicate on the $(x,y)$ -plane the set of all points $(x,y)$ for which $[x]^2 + [y]^2 = 4$ .