Create boosters for training for gold.

Each month, the maximum strength (X) among all citizens is determined and compared with the strength (Y) of each citizen.

The booster increases the power gain for training by K times and is valid for 1 month.

For example:

- If 0 < Y <= 0.3*X, then K = 3;

- If 0.3*X < Y <= 0.5*X then K = 2;

- If 0.5*X < Y <= 0.7*X then K = 1.5;

- If 0.7*X < Y <= 0.9*X then K = 1.2;

- If 0.9*X < Y <= X then K = 1.1;

Or:

- K = 1.1 + 2*(X-Y)/(X+3*Y);