Let n a be the n th term of an A.P. If 100 2 1 r r a = å = a and 100 2 –1 1 β, = å r = r a then the common difference of the A.P. is

Let $a_{n}$ be the $n^{\text {th }}$ term of an A.P. If $\sum_{r=1}^{100} a_{2 r}=\alpha$ and $\sum_{r=1}^{100} a_{2 r-1}=\beta$, then the common difference of the A.P. is