For rolling hash, how do you decide which MOD to pick/ whether to use a tuple of 2 or more hashes? It is hard to know a priori whether the mod is high enough to prevent collisions / too high to cause memory limit / and multiple hashes could cause time limit. In [This](https://codeforces.net/contest/1902/problem/E), [10**9 + 7](https://codeforces.net/contest/1902/submission/235742776) gave an incorrect answer, while [10**18 + 3](https://codeforces.net/contest/1902/submission/235743012) was accepted. Using a pair of hashes with [10**9 + 7 and 998244353](https://codeforces.net/contest/1902/submission/2360318002716) gave memory limit exceeded. ↵
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