The unit digit a 5

Suppose $m$ and $n$ are two integers, not necessarily distinct, from the set $\{1,2,3,...,800\}$ .

What is the probability that the unit digit of $2^m+7^n$ is $5$ ?

If the probability can be expressed as $\dfrac{A}{B}$ for co-prime positive integers $A$ and $B$ , submit the value of $A+B$ as your answer.