so Berland war eternal enemy Flatland Vasya accountant assign fulfil duty nation right situation Berland dismal city surround the army flatlander stand border circle circle center surround city at moment point flatland ring begin quickly direction city that s strategy flatlander usually follow besiege citiesThe berlander sure repel enemys attack learn exact time attack start for need construct radar register movement distance r thus install radar point point enemy ring detect range distance r then radar immediately inform enemys attack due new technology place radar point problem but problem berlander time radar besides large detection radius r radar coststhats Vasyas task task find minimum possible detection radius radar in word task find minimum radius r r0 radar radius r instal point register start movement flatland ring point in problem consider city material point attack enemy ring circle center city radar detection range disk include border center point radar place