#include <iostream>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <algorithm>
#include <string>
#include <vector>
#include <stack>
#include <queue>
#include <set>
#include <map>
#include <unordered_set>
#include <unordered_map>
#include <sstream>
#include <complex>
#include <ctime>
#include <cassert>
#include <functional>

using namespace std;

typedef long long ll;
typedef vector<int> VI;
typedef pair<int,int> PII;

#define REP(i,s,t) for(int i=(s);i<(t);i++)
#define FILL(x,v) memset(x,v,sizeof(x))

const int INF = (int)1E9;
#define MAXN 30005
const int mod = 998244353;
int n, cntq[MAXN], pow26[MAXN], dp[2][MAXN/2], dpPre[MAXN/2];
string s;
int pairchar(int a, int b) {
	if (s[a] == '?' && s[b] == '?') return 26;
	else if (s[a] == '?' || s[b] == '?') return 1;
	else return s[a] == s[b];
}
int ers = 0;
int solve(int l, int r, int pt, int pr) {
	if (l > r) return 1;
	if (dp[pt][l] != -1 && r == pr) {
    return dp[pt][l];
  }
	int ml = n - 1 - l, mr = n - 1 - r;
	int sol = l == r ? pairchar(l, mr) : pairchar(l, mr) * pairchar(r, ml);
	if (sol == 0) return dp[pt ^ 1][l] = 0;
	sol = (ll)sol * solve(l + 1, r - 1, pt, pr) % mod;
	return dp[pt ^ 1][l] = sol;
}
int main() {
	cin >> s;
	n = s.size();
	REP(i,0,n) cntq[i] = (s[i] == '?') + (i ? cntq[i - 1] : 0);
	pow26[0] = 1;
	REP(i,1,n+1) pow26[i] = (ll)pow26[i - 1] * 26 % mod;
	int ans = pow26[cntq[n - 1]];
  int t = 0;
	REP(r,0,n/2) {
    t ^= 1;
    FILL(dp[t], -1);
		dpPre[r] = 0;
		REP(l,0,r+1) {
			dpPre[r] += (ll)solve(l, r, t ^ 1, r - 1) * (l ? dpPre[l - 1] : 1) % mod;
			dpPre[r] %= mod;
		}
		int q = cntq[n - r - 2] - cntq[r];
		ans += (ll)dpPre[r] * pow26[q] % mod;
		ans %= mod;
	}
	cout << ans << endl;
  return 0;
}