1. Added the difficulty prompt when selecting a map

2. It increases the difficulty of map NIGHT - Mutant is completely invisible before being hit, and will show its original shape after being hit, but it is transparent.

3. Fixed the bug of ammunition pickup

4. Changed the gravity of MORNING and DUSK maps

5. The "must see" on the initial page has been added so that players can know more about all aspects of the game, thus reducing the difficulty