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Lost in berlin - a very beautiful problem :p

Source: Brazil Math Olympiad 1998

March 6, 2006
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Problem Statement

Two mathematicians, lost in Berlin, arrived on the corner of Barbarossa street with Martin Luther street and need to arrive on the corner of Meininger street with Martin Luther street. Unfortunately they don't know which direction to go along Martin Luther Street to reach Meininger Street nor how far it is, so they must go fowards and backwards along Martin Luther street until they arrive on the desired corner. What is the smallest value for a positive integer kk so that they can be sure that if there are NN blocks between Barbarossa street and Meininger street then they can arrive at their destination by walking no more than kNkN blocks (no matter what NN turns out to be)?