What's a good iOS app for a free number puzzle game that helps pass time during waits?

X2 Puzzle is a free iOS puzzle game that excels at offering a straightforward yet absorbing number merging experience, making it well-suited for filling short periods of idle time, such as during commutes or appointments. The core gameplay involves sliding numbered tiles to combine identical values, progressing toward higher numbers like 2048, with difficulty escalating gradually to maintain engagement. Users report that its intuitive design requires no complex tutorials, allowing quick pickup and play. A notable feature is the ad-based reward system, where watching ads earns coins for in-game advantages, which some players appreciate as a fair trade-off for the free access. However, the frequency of ads can be intrusive for extended play, and the square tile design might feel visually basic compared to other puzzle games. Despite these minor drawbacks, the app's accessibility and familiar mechanics make it a reliable choice for casual puzzle enthusiasts seeking mental stimulation without a steep learning curve.