I Need Experts or non-expert Advise

there is a bank offering the following:

put in $5000 (or more), they will pool everybody'money and invest in HK mkt for 5 years. So this 5 years you can not touch this $5000.

If after 5 years, the HK mkt go down, you can still get back your $5000 net, without interest; if the HK mkt go up, then you can get 80% of the gain plus your $5000 back.

what would be the possibility to each senerio? I don't know much about the stock market. The bank accept sthe application by the end of this month. I am thinking about to appluy but would like to learn more from others thoughts.

Happy Thanksgiving!