I have this output of xtabs :

```
IsuP<-xtabs(~IsuP+glm_pred_response,data=test)
    glm_predict_response
IsuP  0  1
   0 67 35
   1 54 44

```

I would like to sum up the value of the left upper cell and the value of the right lower cell (67+44) How can I call those cells values only ?
**An update**: How can I get each cell by itself like taking the "35" and add it to "54" or any other combination?