Find the Index of the First Occurrence of the String

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"Given two strings `s` and `p`, return an index `i` of the first occurrence of `p` in `s`. If `p` occurs multiple times, you must return the **smallest** index. If `p` does not occur in `s`, return `-1`. Any leading zeros in `p` should be treated as such, i.e. the function should return correct value if `p` contains leading zeros. "