What is the probability that this problem is interesting?

Probability Level 3

Pi Han randomly chooses two distinct integers from the set of integers $\{1,2,3,4 \dots 99,100\}$ . Let the ratio of the first number to second number chosen be $R$ . If the probability that $R<1$ can be expressed as $\dfrac{a}{b}$ for co-prime integers $a,b$ , Find $a+b$

What is the probability that this problem is interesting?

This problem is original.