∴∠FAG=∠C,
∠∠CFE =∠CEF,
∴∠AGF=∠CFE,
∴AF=AG,
∫∠BAD =∠C,
∴∠BAF=∠DAG,
AB = AD,
∴△ABF≌△ADG,
∴FB=DG,
∴FD+DG=BD,
So the answer is: BD.