homework and exercises - Equivalent resistance of unbalanced Wheatstone Bridge

Equivalent resistance of a balanced Wheatstone Bridge is pretty simple: if the corresponding ratios are equal the just cancel the wire connecting the centre of two wires. But what if they are not equal? What if the cental wire remains in its position? What will be it's equivalent resistance then? I have tried to use the star delta transformation but it is becoming very complex. Can any one of you give a brief formula for calculating the resistance of unbalanced Wheatstone bridge, and of course show it from where it comes? [