// Enjoy your stay.

#include <bits/stdc++.h>

#define long long long
#define LOOPVAR_TYPE long

#define all(x) (x).begin(), (x).end()
#define sz(x) ((LOOPVAR_TYPE)(x).size())
#define foreach(it, X) for(__typeof((X).begin()) it = (X).begin(); it != (X).end(); it++)
#define GET_MACRO(_1, _2, _3, NAME, ...) NAME
#define _rep(i, n) _rep2(i, 0, n)
#define _rep2(i, a, b) for(LOOPVAR_TYPE i = (LOOPVAR_TYPE)(a); i < (LOOPVAR_TYPE)(b); i++)
#define rep(...) GET_MACRO(__VA_ARGS__, _rep2, _rep)(__VA_ARGS__)

#define fir first
#define sec second
#define mp make_pair
#define mt make_tuple
#define pb push_back

const double EPS = 1e-9;
const double PI = acos(-1.0);
const long INF = 1070000000LL;
const long MOD = 1000000007LL;

using namespace std;

typedef istringstream iss;
typedef stringstream sst;
typedef pair<LOOPVAR_TYPE, LOOPVAR_TYPE> pi;
typedef vector<LOOPVAR_TYPE> vi;

string s;

int equal(int a, int b){
	return s[a] == '?' || s[b] == '?' || s[a] == s[b];
}

int main(){
	cin.tie(NULL);
	ios_base::sync_with_stdio(false);

	cin >> s;
	s = "?" + s;
	int n = sz(s);
	int target = (n+1)/2;
	int cur = 0;
	rep(i, (n+1)/2){
		cur += equal(i, n-1-i);
	}
	rep(i, n){
		if(cur == target){
			s[i] = s[n-1-i];
			if(s[i] == '?'){
				s[i] = 'a';
			}
			cout << s << endl;
			return 0;
		}
		if(i == n){
			break;
		}
		cur -= equal(i, n-1-i);
		cur -= equal(i+1, n-1-(i+1));
		swap(s[i], s[i+1]);
		cur += equal(i, n-1-i);
		cur += equal(i+1, n-1-(i+1));
	}
	cout << "NA" << endl;
}