SR graphs and P Matrices

Apologies if something similar has been posted before.

My question is this:

For SR graphs how do we determine the parity of cycles within these graphs i.e. whether a cycle is even or odd?

I have in my notes: 1/2(number of edges in cycle) + number of positive edges in the cycle. I have a sneaking suspicion that I may be wrong.

Also, consider a Matrix, M. Now we know that M is a P matrix i.e. All its principal minors are positive numbers. If we draw the SR graph for M then because it is a P matrix, its SR graph will have no even cycles. Is that right?

Anyone?