Practice · 5 of 6

Final 5: The Lucky Tickets

CHALLENGE

Difficulty: advanced+25 XP

**Bài toán.** Count the integers x with 1 ≤ x ≤ N whose decimal digit sum is divisible by 7. **Constraints:** 1 ≤ N < 10^18 (up to 18 digits). **Input:** one line: N. **Output:** one integer — the count (fits in 64 bits).

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