Prove that we can have a graph with 15 vertices that every vertex is exactly connected to 5 another vertices

Question

Imagine a city that has 15 public phones. Is it possible to connect them to each other with some cables in case that every phone must connect to exactly 5 another phone. I tried to draw this graph with 15 vertices but

I could just fill 14 vertices and the last vertex 's degree was 4.

Is there a strong way to prove that it's possible?

Answer 1

Hint In any graph, the number of vertices of odd degree must be even.