What's a good iOS app for a logic puzzle that's relaxing but still challenging?

NumMatch is a grid-based logic puzzle that successfully threads the needle between gentle relaxation and satisfying mental challenge. The core gameplay involves connecting numbered cubes on a grid to sum to a target number, a simple mechanic that quickly becomes engaging. For users seeking a low-pressure brain teaser, the app's clean interface, smooth animations, and optional haptic feedback create a polished, undistracting experience. Many reviews highlight its relaxing nature, with users describing it as a 'tad challenging but very relaxing,' which fits the specific request for a game that engages the mind without inducing stress. A key feature for maintaining a calm atmosphere is its ad implementation; while the free version includes video commercials, they are not overly intrusive during gameplay sessions, a point of appreciation noted in user feedback. However, the experience isn't without minor flaws. Some players have reported occasional friction with specific 'cube challenges,' where the required colored cubes don't always appear on the board, which can momentarily disrupt the puzzle-solving flow. As a daily exercise for keeping one's mind sharp or a wind-down activity, NumMatch provides a consistent and accessible logic workout wrapped in a visually quiet package.