Esters

Question: Work out the number of esters with the molecular formula C4H8O2. - passerby

My reply: Esters have the functional group R-O-CO-R'. My drawing out the compound and permutating the number of ways the functional group can fit into the compound, this is what I get.
By inserting the functional group between the 3rd and 4th carbon atom will yield the same structural formula as (1). Hence, there're only 2 different esters resulting from the C4H8O2 formula. ;)