Let Tr be the rth term of an A.P. whose first term is a and common difference is d. If for some positive integers 1 mn m n T ,, , m n ¹ = and 1 , Tn m = then a – d equals

Let $T_{\mathrm{r}}$ be the rth term of an A.P. whose first term is a and common difference is $d$. If for some positive integers $m, n, m \neq n, T_{m}=\frac{1}{n}$ and $T_{n}=\frac{1}{m}$, then $a-d$ equals