One day Lucas determined that he had made the smallest number of cubes he could under these conditions. It was a 6-digit number, call it abcdef, but not necessarily implying that there are no duplicate digits.

Lucas decided to make a cryptarithm using his number:

RSTSTS+RTRUTTabcdef

where each of the capital letters represents a distinct digit, the same digit for each occurrence of that letter.

What is RUST?