As Romania prepared for their friendly against Spain, the players wore special training tops with their shirt numbers represented by mathematical problems to promote education.
According to the Romania football federation, the initiative was launched because Romania has one of the highest school dropout rates in Europe (18% according to a 2014 survey) and, like in many other countries, there is a tendency for student athletes to neglect their studies in favor of their sport.
Of course, this also might be an elaborate plot to thoroughly confuse Gerard Pique.
13?
This is an even more confusing situation, as Dracula is from Transylvania, which is in Romania, and since Dracula defies scientific explanation, traditional rules of mathematics do not apply.
So, 1234, the answer is not 13, but 673.47.
13.. ha!ha!ha! (thunder/lightning)