any differences contains both 0 and 1.

Question

we can always assume, if we wish, any differences contains both 0 and 1. $$ $$ A subset $ A=\{a_{1},a_{2},.....,a_{k} \}$ of group $ G$ is called a $ (k,\lambda)$ difference set if for each $ a \neq 0$ the equation $ a_{i}-a_{j}=a $ have exactly $ \lambda $ soltion $ (a_{i},a_{j})$ in A. Further if $ A$ is a difference set , so is the set $ A+i$ for $ i \in G$. But i can't apply it into work. please help me