During the 18th century, one of the greatest mathematicians of all times, Leonhard Euler (1707-1783), lived in Kaliningrad, or Königsberg at the time. He solved a problem known as the "Seven Bridges of Königsberg", which was to decide whether it was possible to find a walk through Königsberg that would cross each of the seven bridges once and only once. Euler figured out that it wasn't possible - and invented a new branch of math, graph theory, along the way. The seven bridges connected the two islands Kant Island (formerly Kneiphof) and October Island (formerly Lomse Island) with the north bank and south bank of Pregel River. Today, there are only five of the seven bridges left (if you count the new highway bridge, Leninskl prospekt, as two), where only two are "originals" from Euler's time (High Bridge and Honey Bridge). Mathematically, that means it's possible today to walk a route, where you cross each of the remaining five bridge once and only once.