#include <iostream>
#include <string>
#include <vector>
using namespace std;

#define MOD 998244353

int main() {
  string s;
  while (cin >> s) {
    vector<int64_t> dyn(s.size()/2+1), mul(s.size()+10, 1);
    dyn[0] = 1;
    int64_t ret = 0;
    for (int y = 0; y < dyn.size(); y++) {
      for (int x = 0; x < y; x++) dyn[y] += (dyn[x] * mul[x+y]) % MOD;
      dyn[y] %= MOD;
      for (int i = 0, j = s.size()-1-y, sum = y+1; i <= y; i++, sum++) {
        if (s[i] != '?' && s[j] != '?' && s[i] != s[j]) mul[sum] = 0;
        if (s[i] == '?' && s[j] == '?') mul[sum] = (mul[sum] * 26) % MOD;
      }
      for (int i = y, j = s.size()-1, sum = y+1; j > s.size()-1-y; j--, sum++) {
        if (s[i] != '?' && s[j] != '?' && s[i] != s[j]) mul[sum] = 0;
        if (s[i] == '?' && s[j] == '?') mul[sum] = (mul[sum] * 26) % MOD;
      }
      int64_t oneWord = dyn[y];
      for (int i = y; i+y < s.size(); i++) if (s[i] == '?') oneWord = (oneWord * 26) % MOD;
      ret += oneWord;
    }
    cout << ret % MOD << endl;
  }
}