import java.io.*;
import java.util.*;

public class prefokery_yuizumi {
	public static void main(String[] args) {
		Scanner cin = new Scanner(System.in);

		while(cin.hasNextLine()) {
			String str = cin.nextLine();
			int n = str.length();

			if(n < 3 || n > 2000) {
				throw new RuntimeException("Illegal Input");
			}

			Operation[][] op = new Operation[n+1][n+1];

			for(int i = 0; i <= 1; i++) {
				for(int j = 0; j + i <= n; j++) {
					op[i][j] = new Create(str.substring(j, j+i));
				}
			}

			for(int i = 2; i <= n; i++) {
				for(int j = 0; j + i <= n; j++) {
					if(str.charAt(j) == str.charAt(j+i-1))
						op[i][j] = new Append(op[i-2][j+1], str.charAt(j));
					else {
						Operation op0 = op[i-1][j+0];
						Operation op1 = op[i-1][j+1];
						op[i][j] = (op0.length() > op1.length()) ? op0 : op1;
					}
				}
			}

			if(op[n][0].length() < 3) {
				throw new RuntimeException("Illegal Input");
			}

			System.out.println(op[n][0].getPalindrome());
			op = null;
			//System.gc();
		}
	}

	private static interface Operation {
		String getPalindrome();
		int length();
	}

	private static class Create implements Operation {
		private String val;

		public Create(String val) {
			this.val = val;
		}

		public String getPalindrome() {
			return val;
		}

		public int length() {
			return val.length();
		}
	}

	private static class Append implements Operation {
		private int        len;
		private Operation  par;
		private char       val;

		public Append(Operation par, char val) {
			this.len = par.length() + 2;
			this.par = par;
			this.val = val;
		}

		public String getPalindrome() {
			return val + par.getPalindrome() + val;
		}

		public int length() {
			return len;
		}
	}
}