Draw squares of sizes 1 by 1 through X by X into the grid, exactly one of each size, so that their vertices lie on grid points and their edges run along grid lines. The edges of these squares do not overlap (they may cross each other), and squares do not share vertices. Each number indicates the sum of the lengths of the edges of all squares that contain the cell with that number.
(Rules (modified) and example (X=4) from WPC 2019 IB)
History of the puzzle
Invented by Vladimir Portugalov (Belarus) in 2006. His first set of puzzle can be seen here.