Let $K_4$ be the complete graph on four vertices. Let $f$ be a continuous map of $K_4$ to the plane such that $f$-images of non-adjacent edges are disjoint. For any vertex $v \in K_4$ take the winding number of the $f$-image of the cycle $K_4 - v$ around $f(v)$. It is known that the sum of these four integers is odd. We construct examples showing that this is the only relation between these four numbers.

翻译：设$K_4$为四个顶点上的完全图。令$f$为$K_4$到平面的连续映射，使得非邻接边的$f$像互不相交。对于任意顶点$v \in K_4$，取循环$K_4 - v$的$f$像围绕$f(v)$的环绕数。已知这四个整数的和为奇数。我们构造实例证明这是这四个数之间唯一的关系。