Vasya came up with his own weather forecasting method. He knows the information about the average air temperature for each of the last n days. Assume that the average air temperature for each day is integral.

Vasya believes that if the average temperatures over the last n days form an arithmetic progression, where the first term equals to the average temperature on the first day, the second term equals to the average temperature on the second day and so on, then the average temperature of the next (n + 1)-th day will be equal to the next term of the arithmetic progression. Otherwise, according to Vasya's method, the temperature of the (n + 1)-th day will be equal to the temperature of the n-th day.

Your task is to help Vasya predict the average temperature for tomorrow, i. e. for the (n + 1)-th day.

Input

The first line contains a single integer n (2 ≤ n ≤ 100) — the number of days for which the average air temperature is known.

The second line contains a sequence of integers t1, t2, ..., tn ( - 1000 ≤ ti ≤ 1000) — where ti is the average temperature in the i-th day.

Output

Print the average air temperature in the (n + 1)-th day, which Vasya predicts according to his method. Note that the absolute value of the predicted temperature can exceed 1000.

Examples

Input

5
10 5 0 -5 -10


Output

-15


Input

4
1 1 1 1


Output

1


Input

3
5 1 -5


Output

-5


Input

2
900 1000


Output

1100

Note

In the first example the sequence of the average temperatures is an arithmetic progression where the first term is 10 and each following terms decreases by 5. So the predicted average temperature for the sixth day is  - 10 - 5 = - 15.

In the second example the sequence of the average temperatures is an arithmetic progression where the first term is 1 and each following terms equals to the previous one. So the predicted average temperature in the fifth day is 1.

In the third example the average temperatures do not form an arithmetic progression, so the average temperature of the fourth day equals to the temperature of the third day and equals to  - 5.

In the fourth example the sequence of the average temperatures is an arithmetic progression where the first term is 900 and each the following terms increase by 100. So predicted average temperature in the third day is 1000 + 100 = 1100.
import java.util.*;
import java.io.*;

/**
 * 18-Sep-2017
 * 11:20:25 PM
 * @author HulkBuster
 * 
 */
public class CF847M {
public static int mod = (int) (1e9 + 7);
public static InputReader in;
public static PrintWriter out;

public static void solve() throws FileNotFoundException {

	in = new InputReader(System.in);
	out = new PrintWriter(System.out);
	int n = in.ni();
	int a = in.ni();
	int b = in.ni();
	
	int d = b-a;
	boolean flag = false;
	int i =2;
	while(i++<n){
		a = b;
		b = in.ni();
		if(d!=(b-a))
			flag = true;
	}
	if(flag)
		out.println(b);
	else
		out.println(b+d);
	out.close();
}

public static void main(String[] args) {
	new Thread(null, new Runnable() {
		public void run() {
			try {
				solve();
			} catch (Exception e) {
				e.printStackTrace();
			}
		}
	}, "1", 1 << 26).start();
}

static class Pair implements Comparable<Pair> {

	int x, y, i;

	Pair(int x, int y, int i) {
		this.x = x;
		this.y = y;
		this.i = i;
	}

	Pair(int x, int y) {
		this.x = x;
		this.y = y;
	}

	public int compareTo(Pair o) {
		if (this.x != o.x)
			return Integer.compare(this.x, o.x);
		else
			return Integer.compare(this.y, o.y);
		//return 0;
	}

	public boolean equals(Object o) {
		if (o instanceof Pair) {
			Pair p = (Pair) o;
			return p.x == x && p.y == y && p.i == i;
		}
		return false;
	}

	public int hashCode() {
		return new Integer(x).hashCode() * 31 + new Integer(y).hashCode() + new Integer(i).hashCode() * 37;
	}

}

public static long add(long a, long b) {
	long x = (a + b);
	while (x >= mod)
		x -= mod;
	return x;
}

public static long sub(long a, long b) {
	long x = (a - b);
	while (x < 0)
		x += mod;
	return x;
}

public static long mul(long a, long b) {
	long x = (a * b);
	while (x >= mod)
		x -= mod;
	return x;
}

public static boolean isPal(String s) {
	for (int i = 0, j = s.length() - 1; i <= j; i++, j--) {
		if (s.charAt(i) != s.charAt(j))
			return false;
	}
	return true;
}

public static String rev(String s) {
	StringBuilder sb = new StringBuilder(s);
	sb.reverse();
	return sb.toString();
}

public static long gcd(long x, long y) {
	if (x % y == 0)
		return y;
	else
		return gcd(y, x % y);
}

public static int gcd(int x, int y) {
	if (x % y == 0)
		return y;
	else
		return gcd(y, x % y);
}

public static long gcdExtended(long a, long b, long[] x) {

	if (a == 0) {
		x[0] = 0;
		x[1] = 1;
		return b;
	}
	long[] y = new long[2];
	long gcd = gcdExtended(b % a, a, y);

	x[0] = y[1] - (b / a) * y[0];
	x[1] = y[0];

	return gcd;
}

public static long mulmod(long a, long b, long m) {
	if (m <= 1000000009)
		return a * b % m;

	long res = 0;
	while (a > 0) {
		if ((a & 1) != 0) {
			res += b;
			if (res >= m)
				res -= m;
		}
		a >>= 1;
		b <<= 1;
		if (b >= m)
			b -= m;
	}
	return res;
}

public static int abs(int a, int b) {
	return (int) Math.abs(a - b);
}

public static long abs(long a, long b) {
	return (long) Math.abs(a - b);
}

public static int max(int a, int b) {
	if (a > b)
		return a;
	else
		return b;
}

public static int min(int a, int b) {
	if (a > b)
		return b;
	else
		return a;
}

public static long max(long a, long b) {
	if (a > b)
		return a;
	else
		return b;
}

public static long min(long a, long b) {
	if (a > b)
		return b;
	else
		return a;
}

public static long pow(long n, long p, long m) {
	long result = 1;
	if (p == 0)
		return 1;
	if (p == 1)
		return n;
	while (p != 0) {
		if (p % 2 == 1)
			result *= n;
		if (result >= m)
			result %= m;
		p >>= 1;
		n *= n;
		if (n >= m)
			n %= m;
	}
	return result;
}

public static long pow(long n, long p) {
	long result = 1;
	if (p == 0)
		return 1;
	if (p == 1)
		return n;
	while (p != 0) {
		if (p % 2 == 1)
			result *= n;
		p >>= 1;
		n *= n;
	}
	return result;
}

public static void debug(Object... o) {
	System.out.println(Arrays.deepToString(o));
}

static class InputReader {
	private final InputStream stream;
	private final byte[] buf = new byte[8192];
	private int curChar, snumChars;
	private SpaceCharFilter filter;

	public InputReader(InputStream stream) {
		this.stream = stream;
	}

	public int snext() {
		if (snumChars == -1)
			throw new InputMismatchException();
		if (curChar >= snumChars) {
			curChar = 0;
			try {
				snumChars = stream.read(buf);
			} catch (IOException e) {
				throw new InputMismatchException();
			}
			if (snumChars <= 0)
				return -1;
		}
		return buf[curChar++];
	}

	public int ni() {
		int c = snext();
		while (isSpaceChar(c)) {
			c = snext();
		}
		int sgn = 1;
		if (c == '-') {
			sgn = -1;
			c = snext();
		}
		int res = 0;
		do {
			if (c < '0' || c > '9')
				throw new InputMismatchException();
			res *= 10;
			res += c - '0';
			c = snext();
		} while (!isSpaceChar(c));
		return res * sgn;
	}

	public long nl() {
		int c = snext();
		while (isSpaceChar(c)) {
			c = snext();
		}
		int sgn = 1;
		if (c == '-') {
			sgn = -1;
			c = snext();
		}
		long res = 0;
		do {
			if (c < '0' || c > '9')
				throw new InputMismatchException();
			res *= 10;
			res += c - '0';
			c = snext();
		} while (!isSpaceChar(c));
		return res * sgn;
	}

	public int[] nIArr(int n) {
		int a[] = new int[n];
		for (int i = 0; i < n; i++) {
			a[i] = ni();
		}
		return a;
	}

	public long[] nLArr(int n) {
		long a[] = new long[n];
		for (int i = 0; i < n; i++) {
			a[i] = nl();
		}
		return a;
	}

	public String readString() {
		int c = snext();
		while (isSpaceChar(c)) {
			c = snext();
		}
		StringBuilder res = new StringBuilder();
		do {
			res.appendCodePoint(c);
			c = snext();
		} while (!isSpaceChar(c));
		return res.toString();
	}

	public String nextLine() {
		int c = snext();
		while (isSpaceChar(c))
			c = snext();
		StringBuilder res = new StringBuilder();
		do {
			res.appendCodePoint(c);
			c = snext();
		} while (!isEndOfLine(c));
		return res.toString();
	}

	public boolean isSpaceChar(int c) {
		if (filter != null)
			return filter.isSpaceChar(c);
		return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
	}

	private boolean isEndOfLine(int c) {
		return c == '\n' || c == '\r' || c == -1;
	}

	public interface SpaceCharFilter {
		public boolean isSpaceChar(int ch);
	}

}
}
