Digit Sum Loop

As an icebreaker, the mentor asks everyone to reduce their student ID number to a single "lucky digit" by repeatedly summing its digits — Fahim needs a program to automate this for the whole batch. Read a positive integer N and repeatedly replace N with the sum of its digits until N becomes a single digit (0-9). Print the final single digit. (This is the "digital root" — try to also think of the O(1) math shortcut using mod 9, not just simulation.)

9875

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