tradingview pine limit

I am trying to write a script on Pine (related to a Tradingview strategy) where I want to use the series and take the previous value or past values to make a set of operations.

Something as:

a = if(Alerta1[-1] == 5, 10,0)

the pine script is saved without error, but during the execution, an error is shown:

Index can’t be a negative value (-1)

Is there anybody that could let me know how to solve it?

I would like to use “[-1]” and other “minus” such “[-20]” for a script.

many thanks in advance