Unit fraction = $\frac{1}{x}$ for some positive integer $x$

Clearly $a,b>1$ . Then $\frac{p-1}{p}=\frac{1}{a}+\frac{1}{b}\leq \frac{1}{2}+\frac{1}{3}=\frac{5}{6}$ . Since $p\geq 5$ , we have $p=5 \implies \frac{p-1}{p}=\frac{4}{5}$ . Note that $\frac{1}{2}+\frac{1}{3} > \frac{4}{5} > \frac{1}{2}+\frac{1}{4}$ . Hence, $\frac{4}{5}$ cannot be expressed as the sum of two distinct unit fractions. Hence answer is $\boxed{-69}$ .