A risk manager gathers five years of historical returns to calculate the Kendall τ correlation coefficient for stocks X and Y. The stock returns for X and Y from 2010 to 2014 are as follows:

| Year | X | Y |

|------|------|-------|

| 2010 | 5.0% | -10.0%|

| 2011 | 50.0%| -5.0% |

| 2012 | -10.0%| 20.0%|

| 2013 | -20.0%| 40.0%|

| 2014 | 30.0% | 15.0%|

What is the Kendall τ correlation coefficient for the stock returns of X and Y?