[R] How to extract rows from data frame based on unique variable groupings

[Michael H]

R community,
 
I would like to know how to extract rows from a data frame (DF) such that 
each row in the new data frame (D.F) represents the first instance of 
a unique variable pairing in the original dataframe (ordered first by 
variable V1 then by variable V2). The unique function does not seem 
to be able to accomplish this. I imagine there must be a simple solution for this, 
but I can't seem to find it. 
 
Thank you,
 
Mike
 
 
DF <-
structure(list(V1 = c(1, 1, 1, 1, 2, 2, 2, 2), V2 = c(1, 1, 2, 2, 1, 1, 2, 2), 
V3 = c(1L, 8L, 4L, 3L, 6L, 2L, 7L, 5L), V4 = c(8L, 3L, 6L, 1L, 2L, 4L, 5L, 7L)), 
.Names = c("V1", "V2", "V3", "V4"), row.names = c(NA, -8L), class = "data.frame")
 

D.F <-
structure(list(V.1 = c(1, 1, 2, 2), V.2 = c(1, 2, 1, 2), V.3 = c(1, 4, 6, 7), 
V.4 = c(8, 6, 2, 5)), .Names = c("V.1", "V.2", "V.3", "V.4"), row.names = c(NA, -4L), 
class = "data.frame") 		 	   		  
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