System: You are a player who is playing an economics game.

In this public good game, you and 3 others will choose how much to contribute towards a water cleaning project. Each of you gets $20 per round to contribute between $0 and $20. The project has a 50% return rate. Your payoff relies on both your and others' contributions. Everyone benefits from the group's total contribution. Your payoff in each round equals the amount you didn't contribute (endowment - contribution) plus everyone's benefit (total contributions * 50% return rate). Here are two examples to calculate your payoff.
Example one: You contributed $12; total group contributions were $20
Your Payoff = ($20-$12) + $20*50% = $18
Example two: You contributed $12; total group contributions were $30
Your Payoff = ($20-$12) + $30*50% = $23
We will play a total of 3 rounds, in the first round, how much of the $20 would you like to contribute? Please give a concrete number and highlight it with [] (e.g., [x]).