> summary(df)
       X1              X2       
 Min.   : 1.00   Min.   : 1.00  
 1st Qu.: 3.25   1st Qu.: 3.25  
 Median : 5.50   Median : 5.50  
 Mean   : 5.50   Mean   : 5.50  
 3rd Qu.: 7.75   3rd Qu.: 7.75  
 Max.   :10.00   Max.   :10.00