You are researching the penguin population in the Falkland Islands. There are 1000 penguins in the beginning of the first year: 500 are male, and 500 are female. Each couple makes one chick every spring (assuming every penguin has a partner). Chicks that are one year old also reproduce. However, 20% of all penguins, both adults and chicks, die each year due to natural causes. You need to create a formula to determine the population of the penguins (including chicks) at the end of the 7th year. What is the formula to determine the population of the penguins at the end of the 7th year?