import java.io.*;

public class MadamImAdam_MemoizedDFS_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;

    private static int[][] length;
    private static int[][] direct;
    private static String line;

    private static int DFS(int left, int right) {
        if (length[left][right] > 0) { return length[left][right]; }
        if (left > right) { return 0; }

        int result, result1, result2;

        result1 = DFS(left+1, right);
        result2 = DFS(left, right-1);
        if (result1 > result2) { result = result1; direct[left][right] = RIGHT; }
        else                   { result = result2; direct[left][right] = UP; }

        if (line.charAt(left) == line.charAt(right)) {
            int result3 = DFS(left+1, right-1) + 2;
            if (result3 > result) { result = result3; direct[left][right] = DIAG; }
        }

        length[left][right] = result;
        return result;
    }

    private static int Solve() {
        length = new int[line.length()][line.length()];
        direct = new int[line.length()][line.length()];
        for (int i = 0; i < line.length(); i++) {
            for (int j = 0; j < line.length(); j++) {
                if (i == j) {
                    length[i][j] = 1;
                } else {
                    length[i][j] = 0;
                }
                direct[i][j] = NODIRECTION;
            }
        }
        return DFS(0, line.length()-1);
    }

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

        while (null != (line = br.readLine())) {
            Solve();

            /* 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);
            }
        }
    }
}