I have two values 3 and 4. I would like to add these numbers to A1 alternately as I copy down the previous cell only. So lets A1=1, in the first copy down A2 = A1+3, and the next copy down A3=A2+4, and the next A4=A3+3, etc. it will go on like this. How can we formulate this?

A1
A2=A1+3
A3=A2+4
A4=A3+3
A5=A4+4
...
etc..