#include <iostream>
#include <algorithm>
#include <vector>
#include <string>

void solve() {
    std::string s;
    std::cin >> s;
    int n = s.length();

    std::string fore, back;

    auto dfs = [&](auto&& f, int l, int r, bool rem) -> bool {
        if (l == r) fore.push_back(s[l++]);
        if (l > r) {
            if (rem) back.push_back(s[r]);
            return true;
        }

        if (s[l] == s[r]) {
            if (f(f, l + 1, r - 1, rem)) {
                fore.push_back(s[l]);
                back.push_back(s[r]);
                return true;
            }
        }

        if (rem) {
            if (f(f, l, r - 1, false)) {
                fore.push_back(s[r]);
                back.push_back(s[r]);
                return true;
            }
            if (f(f, l + 1, r, false)) {
                fore.push_back(s[l]);
                back.push_back(s[l]);
                return true;
            }
        }

        return false;
    };

    if (!dfs(dfs, 0, n - 1, true)) {
        std::cout << "NA\n";
    } else {
        std::reverse(fore.begin(), fore.end());
        std::cout << fore << back << "\n";
    }
}

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

    solve();

    return 0;
}