Whether AUC can be calculated as average of sensitivity and specificity or not? I have gone through few of the paper for AUC calculation, following paper has mentioned AUC can be calculated as

AUC = (Sensitivity +Specificity)/2. 

A systematic analysis of performance measures for classification tasks

Please let me know that whether it is correct or not.
The article you mentioned is referring to the AUC of Receiver Operating Characteristic Curve (ROC-Curve), that is different from the AUC of the Precision-Recall Curve (PR-Curve). See this post to know about the differences between the PR-Curve and the ROC-Curve. If what you want is that same value for the PR-Curve you should look at the F1-Score, see this wikipedia article for an explanation of the relation between the AUC of the ROC-Curve, the Youden Index and the F1-Score. Also a good indicator of the AUC of the PR-Curve is the Average Precision (AP) as explained here.