{"problem":{"name":"Both Reversible","description":{"content":"A string $T$ is called a **good string** when it satisfies the following condition: *   There is a pair of strings $(A, B)$ that satisfies all of the following:     *   Both $A$ and $B$ are non-empty","description_type":"Markdown"},"platform":"AtCoder","limit":{"time_limit":2000,"memory_limit":262144},"difficulty":"None","is_remote":true,"is_sync":true,"sync_url":null,"sign":"arc171_f"},"statements":[{"statement_type":"Markdown","content":"A string $T$ is called a **good string** when it satisfies the following condition:\n\n*   There is a pair of strings $(A, B)$ that satisfies all of the following:\n    *   Both $A$ and $B$ are non-empty.\n    *   $A + B = T$.\n    *   Both $A + \\mathrm{rev}(B)$ and $\\mathrm{rev}(A) + B$ are palindromes.\n\nHere, $A + B$ denotes the string formed by concatenating strings $A$ and $B$ in this order.  \nAlso, $\\mathrm{rev}(A)$ denotes the string formed by reversing the order of the characters in string $A$.\nThere is a string $S$ of length $N$ consisting of lowercase English letters and the character `?`.  \nAmong the $26^{(\\text{number of ?s})}$ ways to replace the `?`s in $S$ with lowercase English letters, how many result in a good string? Find the count modulo $998244353$.\n\n## Constraints\n\n*   $2 \\leq N \\leq 5 \\times 10^4$\n*   $S$ is a string of length $N$ consisting of lowercase English letters and `?`.\n\n## Input\n\nThe input is given from Standard Input in the following format:\n\n$N$\n$S$\n\n[samples]","is_translate":false,"language":"English"}],"meta":{"iden":"arc171_f","tags":[],"sample_group":[["4\n?ba?","1\n\nThe string `abab` is good, because if we set $A =$ `ab` and $B =$ `ab`, then $A + B =$ `abab`, and both $A + \\mathrm{rev}(B) =$ `abba` and $\\mathrm{rev}(A) + B =$ `baab` are palindromes.  \nAmong the strings that can be formed by replacing the `?`s in $S$ with lowercase English letters, there is only one good string, which is `abab`."],["10\n?y?x?x????","676"],["30\n???a?????aab?a???c????c?aab???","193994800"],["36\n????????????????????????????????????","363594614"]],"created_at":"2026-03-03 11:01:13"}}