#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>

#define FORR(i,b,e) for(int i=(b);i<(int)(e);++i)
#define FOR(i,e) FORR(i,0,e)
#define sortuniq(v) sort(v.begin(), v.end()), v.erase(unique(v.begin(),v.end()),v.end())
#define dump(var) cerr << #var ": " << var << "\n"
#define dumpc(con) for(auto& e: con) cerr << e << " "; cerr<<"\n"

typedef long long ll;
typedef unsigned long long ull;

const double EPS = 1e-6;
const int d4[] = {0, -1, 0, 1, 0};

using namespace std;


int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);

    string s;
    cin >> s;

    bool na = false;
    int ri = -1, l = s.size();
    int ofl = 0, ofr = 0;
    char rc;
    FOR(i, l/2) {
        if (s[i+ofl] != s[l-1-i-ofr]) {
            if (ri >= 0) {
                na = true;
                break;
            }
            if (s[i+ofl] == s[l-1-i-ofr-1]) {
                rc = s[l-1-i-ofr];
                ri = i+ofl;
                ofr = 1;
            }
            else if (s[i+ofl+1] == s[l-1-i-ofr]) {
                rc = s[i+ofl];
                ri = l-1-i-ofr+1;
                ofl = 1;
            }
            else {
                na = true;
                break;
            }
        }
    }

    if (!na && ri < 0) {
        if (l % 2 == 0) {
            ri = l/2;
            rc = 'x';
        }
        else if (s[0] == s[l/2] && s[0] == s[max(l-2, 0)]) {
            ri = 0;
            rc = s[0];
        }
        else {
            na = true;
        }
    }
    if (na) cout << "NA" << endl;
    else cout << s.substr(0, ri) << rc << s.substr(ri) << endl;

    return 0;
}