Draw a closed loop passing through the centres of all cells horizontally and vertically. The numbers in a thickly outlined region indicate the highest number of cells the loop passes consecutively in that area. This number must be achieved at least once.
(Rules and example from WPC 2017 IB)
History of the puzzle
Invented by Naoki Inaba (Japan) in 2003 under the name MAXライン (MAX Line). First set of puzzles he created can be seen on his website. 
Appearances in the past WPCs
- WPC 2017/Round 16 by Ashish Kumar