package chelper;

import io.InputReader;
import io.OutputWriter;
import misc.SimpleSavingChelperSolution;


public class M extends SimpleSavingChelperSolution {

  public void solve(int testNumber, InputReader in, OutputWriter out) {
    wrapSolve(testNumber, in, out);
  }

  @Override
  public void solve(int testNumber) {
    int n = in.nextInt();

    int[] a = in.nextIntArray(n);

    boolean ok = true;
    int d = a[1] - a[0];

    for (int i = 0; i < n - 1; i++) {
      int t = a[i + 1] - a[i];

      ok &= t == d;
    }

    out.println(ok ? (a[n - 1] + d) : a[n - 1]);
  }
}