IMO Shortlist 1969 problem 60
Dodao/la:
arhiva2. travnja 2012. 
Find the natural number

with the following properties:

Let

be an arbitrary finite set of points in the plane, and

the distance from

to the origin

We assign to each

the closed disk

with center

and radius

. Then some

of these disks contain all points of

is the smallest integer with the above property.
%V0
$(SWE 3)$ Find the natural number $n$ with the following properties:
$(1)$ Let $S = \{P_1, P_2, \cdots\}$ be an arbitrary finite set of points in the plane, and $r_j$ the distance from $P_j$ to the origin $O.$ We assign to each $P_j$ the closed disk $D_j$ with center $P_j$ and radius $r_j$. Then some $n$ of these disks contain all points of $S.$
$(2)$ $n$ is the smallest integer with the above property.
Izvor: Međunarodna matematička olimpijada, shortlist 1969