#include <iostream>
#include <string>

using namespace std;

const int MOD = 998244353;
int dp[15005];
int getAmt(char a, char b) {
  if(a == '?' && b == '?') return 26;
  if(a == '?' || b == '?') return 1;
  if(a == b) return 1;
  return 0;
}
int n;
string s;
void force(int la, int ra, int lb, int rb) {
  int scale = 1;
  while(la >= 0 && ra < lb && rb < n) {
    scale = (scale * (long long)getAmt(s[la], s[lb])) % MOD;
    if(la != ra) scale = (scale * (long long)getAmt(s[ra], s[rb])) % MOD;
    if(scale == 0) break;
    dp[(rb-la+1)/2] += (scale * (long long)dp[(lb-ra)/2]) % MOD;
    if(dp[(rb-la+1)/2] >= MOD) dp[(rb-la+1)/2] -= MOD;
    la--; lb--;
    ra++; rb++;
  }
}

int main() {
  cin >> s;
  int origs = 1;
  if(s.size() % 2) {
    int mid = s[s.size()/2];
    if(mid == '?') origs = 26;
    s = s.substr(0, s.size()/2) + s.substr(s.size()/2 + 1);
  }
  dp[0] = 1;
  n = s.size();
  {
    int li = n/2-1;
    int ri = n/2;
    int curr = 1;
    int ni = 1;
    while(li >= 0 && ri < n && curr) {
      curr = (curr * (long long)(s[li--] == '?' ? 26 : 1)) % MOD;
      curr = (curr * (long long)(s[ri++] == '?' ? 26 : 1)) % MOD;
      dp[ni++] = curr;
    }
  }
  for(int lhs = 0; lhs < n/2; lhs++) {
    int li = (n/2-1) - (lhs);
    int ri = li + 2 * lhs + 1;
    force(li, li, ri, ri);
    force(li-1, li, ri, ri+1);
  }
  int ans = (origs * (long long)dp[n/2]) % MOD;
  cout << ans << "\n";
}