Return the nth rational number.
This is the inverse to Problem 1471. Index of a Rational number
The first ten positive rational numbers under the enumeration using cantors diagonalization argument are 1/1, 2/1, 1/2, 3/1, 2/2, 1/3, 4/1, 3/2, 2/3, 1/4.
Find the index of a positive rational number enumerated in this manner, i.e. the 6th rational number is 1/3.