Let ${\mathrm{t}}_{\mathrm{r}}$ 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 $\left(\mathrm{m}\ne \mathrm{n}\right) {\mathrm{T}}_{\mathrm{m}}=\frac{1}{\mathrm{n}} \mathrm{and} {\mathrm{T}}_{\mathrm{n}}=\frac{1}{\mathrm{m}}, \mathrm{then} \mathrm{a}-\mathrm{d}=$