In Königsberg, there was a problem with the royal bridges on Pregolya.

An excerpt from the article 24 facts about Kaliningrad

This problem was solved by a Swiss mathematician and physicist, a pioneer in many areas of these sciences - Leonhard Euler, who spent most of his life in Russia and Prussia.

The Pregolya river flowing through Królewiec had two islands in its forks. Seven bridges were thrown over the river, one of which connected both islands, and the remaining bridges connected the islands with the banks of the river.

Euler, using graph theory, solved the problem of the bridges in Königsberg, making it possible to cross all the bridges in turn so that each bridge was crossed only once.

This issue was described by Euler in 1741 in his work Solutio problematis ad geometriam situs pertinentis in Commentarii academiae scientiaru, which is considered the first work on graph theory.