Trouble Calculating DC Resistance

I was looking through some old electronics books of mine, and I can't
seem to figure out the equation to calculate the following circuit:

o
|
|
---------
| |
R R
1 4
| |
---R 3---
| |
R R
2 5
| |
---------
|
|
o

I just can't figure out how R3 fits in. I would guess that it is
somehow in parallel, but I'm not sure with what. Could someone write
out the equation for solving the total resistance of this circuit? I'm
very confused.

Thanks a bunch,

Danny H.