Best Method to Calulate Average of Data in Column 2 based on EVEN / ODD Status of Column 1

Howdy-
I have data (ID #) listed in whole integers {Column 1} and Days until treatment intitiated in {Column 2} from a trial where EVEN ID's received treatment 1 and ODD Id's rec'd treatment 2. I am trying to calculate averages by treatment group ODD or EVEN and having trouble.

ID Days
18893 3
18895 3
18880 5
18771 6
18836 6
18855 7
18767 13
18773 14
18780 15

Thanks Marvin-
That will get me going again-