accuracy on a matrix

[Carlo Rossi]

Hello,
 I have a problem that is not strictly on Octave but maybe it can be interesting as I didn't find solution anywhere.
I have a matrix where each column/rows represent a class; I'm speaking about a confusion matrix.
for example, three classes conf. matrix
A = [2 1 1; 0 3 1; 0 0 4];

and I read this: http://en.wikipedia.org/wiki/Accuracy_and_precision
Is there any chance to use the first formula of accuracy (actually with more than 2 classes I don't understand how apply it) without use the Prevalence,Sensitivity etc?

thanks,