Everybody knows what an arithmetic progression is. Let us remind you just in case that an arithmetic progression is such sequence of numbers a1, a2, ..., an of length n, that the following condition fulfills: 

a2 - a1 = a3 - a2 = a4 - a3 = ... = ai + 1 - ai = ... = an - an - 1.

For example, sequences [1, 5], [10], [5, 4, 3] are arithmetic progressions and sequences [1, 3, 2], [1, 2, 4] are not.

Alexander has n cards containing integers. Arthur wants to give Alexander exactly one more card with a number so that he could use the resulting n + 1 cards to make an arithmetic progression (Alexander has to use all of his cards).

Arthur has already bought a card but he hasn't written a number on it. Help him, print all integers that you can write on a card so that the described condition fulfilled.

Input

The first line contains integer n (1 ≤ n ≤ 105) — the number of cards. The next line contains the sequence of integers — the numbers on Alexander's cards. The numbers are positive integers, each of them doesn't exceed 108.

Output

If Arthur can write infinitely many distinct integers on the card, print on a single line -1.

Otherwise, print on the first line the number of integers that suit you. In the second line, print the numbers in the increasing order. Note that the numbers in the answer can exceed 108 or even be negative (see test samples).

Examples

Input

3
4 1 7


Output

2
-2 10


Input

1
10


Output

-1


Input

4
1 3 5 9


Output

1
7


Input

4
4 3 4 5


Output

0


Input

2
2 4


Output

3
0 3 6
//    A Computer is Like a mischievous genie.
//    It will give you exactly what you ask for,
//    but not always what you want
//    A code by Rahul Verma


import java.io.BufferedOutputStream;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedHashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.PriorityQueue;
import java.util.Queue;
import java.util.Set;
import java.util.Stack;
import java.util.StringTokenizer;
import java.util.TreeMap;
import javax.xml.soap.SAAJResult;


public class Main {


    static class Clock {

        protected long start, stop;

        public void start() {

            start = System.currentTimeMillis();
        }

        public void stop() {
            stop = System.currentTimeMillis();
        }

        public String getTime() {

            return ((stop - start) + " ms");
        }
    }


    public static PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out));

    static class FastReader {

        BufferedReader br;
        StringTokenizer st;

        public FastReader() {
            br = new BufferedReader(new
                InputStreamReader(System.in));
        }

        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        int nextInt() {
            return Integer.parseInt(next());
        }

        long nextLong() {
            return Long.parseLong(next());
        }

        double nextDouble() {
            return Double.parseDouble(next());
        }

        String[] nextSArray() {
            String sr[] = null;
            try {
                sr = br.readLine().trim().split(" ");
            } catch (IOException e) {
                e.printStackTrace();
            }
            return sr;
        }

        String nextLine() {
            String str = "";
            try {
                str = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }

            return str;
        }
    }


    static long powmodulo(long a, long p) {
        if (p == 0) {
            return 1 % mod;
        }
        if (p == 1) {
            return a % mod;
        }
        long ans = 1;
        while (p > 0) {
            if ((p & 1) > 0) {
                ans = (ans * a) % mod;
            }
            a = (a * a) % mod;
            p = p >> 1;
        }
        return ans % mod;
    }


    static long mod = 1000000007;

    static long gcd(long a, long b) {
        if (a == 0) {
            return b;
        }
        return gcd(b % a, a);
    }

    static long fast_powerNumbers(long a, long n) {
        if (n == 1) {
            return a;
        }
        long ans = fast_powerNumbers(a, n / 2);
        if (n % 2 == 0) {
            return (ans * ans);
        } else {
            return ((ans * ans) * (a));
        }
    }


    static void dfs_helper(int[][] arr, int i, int j, int team, int n, int m) {
        arr[i][j] = team;
        if (i - 1 >= 0 && arr[i - 1][j] == 1) {
            dfs(arr, i - 1, j, team, n, m);
        }
        if (j - 1 >= 0 && arr[i][j - 1] == 1) {
            dfs(arr, i, j - 1, team, n, m);
        }
        if (i + 1 < n && arr[i + 1][j] == 1) {
            dfs(arr, i + 1, j, team, n, m);
        }
        if (j + 1 < m && arr[i][j + 1] == 1) {
            dfs(arr, i, j + 1, team, n, m);
        }

    }

    static void dfs(int[][] arr, int i, int j, int team, int n, int m) {
        dfs_helper(arr, i, j, team, n, m);

    }
    public static int arr[];


    static void seive(int n) {
        // 1 is not prime
        arr = new int[n + 1];
        arr[0] = arr[1] = 1;
        for (int i = 4; i <= n; i = i + 2) {
            arr[i] = 1;
        }
        for (int i = 3; i * i <= n; i = i + 2) {
            if (arr[i] == 0) {
                for (int j = i * i; j <= n; j = j + i) {
                    arr[j] = 1;
                }

            }
        }
    }






    public static void main(String[] args) throws Exception {
        FastReader sc = new FastReader();

        Clock clock = new Clock();

        clock.start();
        int n=sc.nextInt();
        long arr[]=new long[n];
        long key1=Long.MAX_VALUE;
        long key2=Long.MAX_VALUE;
        int c1=0;
        int c2=0;
        for (int i = 0; i < n; i++) {
        arr[i]=sc.nextLong();
        }
        if(n==1)
        {
            System.out.println(-1);
            return;
        }

        Arrays.sort(arr);
        key1=arr[1]-arr[0];
        c1++;
        for (int i = 2; i < n; i++) {
            long d = arr[i]-arr[i-1];
            if(d==key1)
            {
                ++c1;
            }
            else if(key2==d)
            {
                ++c2;
            }
            else if(key2==Long.MAX_VALUE)
            {
                key2=d;
                ++c2;
            }
            else
            {
                System.out.println(0);
                return;
            }
            if(c1>1&&c2>1)
            {
                System.out.println(0);
                return;
            }


        }
//        System.out.println(c1+" "+key1);
//        System.out.println(c2+" "+key2);
        if(c2==0&&key1==0)
        {
            System.out.println(1);
            System.out.println(arr[0]);
            return;
        }
        if(n==2)
        {
            if ((arr[1] - arr[0]) % 2 == 0) {
                System.out.println(3);
                long d=arr[1]-arr[0];
                System.out.println((arr[0]-d)+" "+((arr[0]+arr[1])/2)+" "+(arr[1]+d));
            } else {
                System.out.println(2);
                long d=arr[1]-arr[0];
                System.out.println((arr[0]-d)+" "+(arr[1]+d));
            }
            return;
        }
        if(c2==0)
        {
            System.out.println(2);
            System.out.println((arr[0]-key1)+" "+(arr[n-1]+key1));
        }
        else
        {
            if( (key1/2.0==key2 && c1==1 )|| (key2/2.0==key1 && c2==1))
            {
                System.out.println(1);
            }
            else
            {
                System.out.println(0);
                return;
            }
            long key=0;
            if(key1/2.0==key2)
                key=key1;
            else
                key=key2;
            //System.out.println(key);
            for (int i = 1; i <n ; i++) {
                long d=arr[i]-arr[i-1];
                if(d==key)
                {
                    System.out.println((arr[i]+arr[i-1])/2);
                }

            }

        }



    }



}


class Pair{

    long a,b;
    Pair(long a,long b)
    {
        this.a=a;
        this.b=b;
    }
}
class DSU
{
    int parent[];
    int rank[];

    long sum[] ;
    DSU(int n)
    {
        this.parent=new int[n];
        this.rank=new int[n];
        sum=new long[n];
        Arrays.fill(parent,-1);
        Arrays.fill(rank,1);

    }

    int find(int s1)
    {
        if(parent[s1]==-1)
            return s1;
        return parent[s1]=find(parent[s1]);
    }

    void unite(int s1,int s2)
    {
        int p1=this.find(s1);
        int p2=this.find(s2);
        if(p1==p2)
            return;
        if(rank[p1]>rank[p2])
        {
            parent[p2]=p1;
            rank[p1]+=rank[p2];
            // System.out.println(arr[p2]);

            sum[p1]+=sum[p2];
            //sum[p2]+=sum[p1];

        }
        else
        {
            parent[p1]=p2;
            rank[p2]+=rank[p1];
            //System.out.println(arr[p1]);
            sum[p2]+=sum[p1];
            //sum[p1]+=sum[p2];
        }
    }


}

class Gaph {

    HashMap<Integer, ArrayList<Integer>> hm;

    Gaph() {
        hm = new HashMap<>();
    }

    Gaph(int n) {

        hm = new HashMap<>();
        for (int i = 0; i < n; i++) {
            hm.put(i, new ArrayList<Integer>());
        }
    }

    // function for adding an edge.................................................
    public void addEdge(int a, int b, boolean isDir) {
        if (isDir) {
            if (hm.containsKey(a)) {
                hm.get(a).add(b);
            } else {
                hm.put(a, new ArrayList<>(Arrays.asList(b)));
            }
        } else {
            if (hm.containsKey(a)) {
                hm.get(a).add(b);
            } else if (!hm.containsKey(a)) {
                hm.put(a, new ArrayList<>(Arrays.asList(b)));
            }

            if (hm.containsKey(b)) {
                hm.get(b).add(a);
            } else if (!hm.containsKey(b)) {
                hm.put(b, new ArrayList<>(Arrays.asList(a)));
            }
        }
    }


}

// out.println(al.toString().replaceAll("[\\[|\\]|,]",""));