TI 84 programming

For a ti-84 program I used the quadratic equation and stored the root with the plus sign as N and the root with the negative sign as R. I then want to only use the positive root as the value for the rest of the program (if both are positive it doesn't matter I just want to check one is positive), so I did as follows:

If N>0 --> U

ElseR-->U

But it didn't work. Is it not possible to use store as a command in an if-else statement? Or is there another way to only choose the positive variable and store that as U?

Thanks in advance