67 sequence 1

If a character $6$ or $7$ remains in the string, the condition is not satisfied. Therefore, every occurrence of $6$ and $7$ in the original string must be erased.

Conversely, if we erase exactly all occurrences of $6$ and $7$, the remaining string contains neither $6$ nor $7$. Hence the answer is the number of occurrences of the characters $6$ and $7$ in $S$.

We can compute it by scanning the string once. The time complexity is $O(N)$ and the memory complexity is $O(1)$.