need calculation to add sums starting one less than starting number

I am trying to find a formula that will automatically add the sum.

For example let's say I start with 10. I want to start one number below 10 & add them all up. For example, if starting number is 10 then I would take: 9+8+7+6+5+4+3+2+1. I need to be able to do this for any starting number.

If I start at 100 it would be 99+98+97+96+96+94 all the way down to zero. If starting at 56, I would need to start adding 55+54+53+52+51+50 all the way to zero.

Do you know a formula I can use? The starting number can be any number, but I need to start one number below the starting numbers and add each following number.