import java.io.*;

public class MadamImAdam_N2_dmikurube {
    private static final int NODIRECTION = 0;
    private static final int RIGHT = 1;
    private static final int UP = 2;
    private static final int UPRIGHT = 3;
    private static final int DIAG = 4;

    public static void main(String[] args) throws IOException {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String line;

        while (null != (line = br.readLine())) {
            int[][] length = new int[line.length()][line.length()];
            int[][] direct = new int[line.length()][line.length()];
            for (int l = 0; l < line.length(); l++) {
                for (int r = 0; r < line.length(); r++) {
                    length[l][r] = 0;
                    direct[l][r] = NODIRECTION;
                }
            }
            for (int lr = 0; lr < line.length(); lr++) {
                length[lr][lr] = 1;
            }

            for (int r = 0; r < line.length(); r++) {
                for (int l = r-1; l >= 0; l--) {
                    if (line.charAt(l) == line.charAt(r)) {
                        length[l][r] = length[l+1][r-1] + 2;
                        direct[l][r] = DIAG;
                    } else {
                        if (length[l+1][r] == length[l][r-1]) {
                            length[l][r] = length[l+1][r];
                            direct[l][r] = UPRIGHT;
                        } else if (length[l+1][r] > length[l][r-1]) {
                            length[l][r] = length[l+1][r];
                            direct[l][r] = RIGHT;
                        } else {
                            length[l][r] = length[l][r-1];
                            direct[l][r] = UP;
                        }
                    }
                }
            }

            /* It's so shoddy. */
            {
                String result_left = "", result_right = "", result = "";
                int l = 0, r = line.length() - 1;
                while (true) {
                    if (direct[l][r] == NODIRECTION) {
                        if (length[l][r] == 1) { result = result_left + line.charAt(l) + result_right; }
                        else { result = result_left + result_right; }
                        break;
                    } else if (direct[l][r] == DIAG) {
                        result_left = result_left + line.charAt(l);
                        result_right = line.charAt(r) + result_right;
                        l++; r--;
                    } else if (direct[l][r] == UP) {
                        r--;
                    } else {
                        l++;
                    }
                }
                System.out.println(result);
            }
        }
    }
}