We have the following function that generates the flag
We can see that it is called in main() if the following condition is met:
If we check constant definitions at the top of the file we find that victory_tile = pygame.Vector2(10, 10).
So looking back at GenerateFlagText we know that it takes the x,y coordinates of the player which will be 10,10 which we can infer from the code. The key would then be 10+10*20 which is 210.
Alternate solution
We have the block class which has the parameter passable.
We also have a huge array of blocks declared in BuildBlocks()
Looking at the array we can see that 2 blocks have passable set to True
Keep in mind in pygame 0,0 is The top left of the window