Every day, Lucas receives a cube of metal whose side is between 9.5 and 10.5 meters. He melts the cube down and uses the resulting metal to make smaller cubes, whose sides are all between 9.5 and 10.5 centimeters, leaving no metal left over.

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
+RTRUTT
 abcdef

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

What is RUST?