Slitherlink Puzzles - 200 Very Hard Puzzles 25x25 Book 20

Slitherlink is a logic puzzle is played on a rectangular lattice of dots. Some of the squares formed by the dots have numbers inside them. The objective is to connect horizontally and vertically adjacent dots so that the lines form a simple loop with no loose ends. In addition, the number inside a square represents how many of its four sides are segments in the loop.