hi was wondering if the is anyway to auto fill using a formula

this is what i want to do i have a loan that i pay x amount every month it is over 6 years.
so i have auto filled every month for the next six years now i want to fill in the formula so that it show the outstanding amount left to pay that month so for example lets say i have 5000 in a1 column to start and pay 100 per month. in a2 i have put the formula =a1-100
then in a3 i have put =a2-100 so is there anyway to auto fill this as i dont want to type it 60 times thank you in advance for your help