If you put $1 a day away and every day you add a dollar to the previous days amount, how much would

If you put $1 a day away and every day you add a dollar to the previous days amount, how much would you have after 100 days

Day 1, you have 1
Day 2, you have 1 + 1 = 2
Day 3, you have 1 + 2 = 3

So our formula for day n is:
D(n) = n

D(100) = 100