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
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.util.ArrayDeque;
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.LinkedList;
import java.util.List;
import java.util.PriorityQueue;
import java.util.Queue;
import java.util.Random;
import java.util.Set;
import java.util.Stack;
import java.util.StringTokenizer;
import java.util.TreeSet;
import java.util.stream.Collectors;

public class Main {
    static int mod = (int) (1e9) + 7;

    /* ======================DSU===================== */
    static class dsu {
        static int parent[], n;// min[],value[];
        static long size[];

        dsu(int n) {
            parent = new int[n + 1];
            size = new long[n + 1];
            // min=new int[n+1];
            // value=new int[n+1];
            this.n = n;
            makeSet();
        }

        static void makeSet() {
            for (int i = 1; i <= n; i++) {
                parent[i] = i;
                size[i] = 1;
                // min[i]=i;
            }
        }

        static int find(int a) {
            if (parent[a] == a)
                return a;
            else {
                return parent[a] = find(parent[a]);// Path Compression
            }
        }

        static void union(int a, int b) {
            int setA = find(a);
            int setB = find(b);
            if (setA == setB)
                return;
            if (size[setA] >= size[setB]) {
                parent[setB] = setA;
                size[setA] += size[setB];
            } else {
                parent[setA] = setB;
                size[setB] += size[setA];
            }
        }
    }

    /* ======================================================== */
    static class Pair implements Comparator<Pair> {
        long lx, ly;
        int ix, iy;

        // Constructor
        public Pair(long x, long y) {
            // System.out.println("F1");
            this.lx = x;
            this.ly = y;
            this.ix = (int) x;
            this.iy = (int) y;
        }

        public Pair(int x, int y) {
            // System.out.println("F");
            this.ix = x;
            this.iy = y;
            this.lx = ix;
            this.ly = iy;
            // System.out.println(this.lx);
        }

        public Pair() {
        }

        @Override
        public int compare(Main.Pair o1, Main.Pair o2) {
            // System.out.println( (o1.iy - o2.iy));
            return ((int) (o1.ix - o2.ix));// Ascending Order
        }
    }

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

        int[] intArr(int n) {
            int res[] = new int[n];
            for (int i = 0; i < n; i++)
                res[i] = nextInt();
            return res;
        }

        long[] longArr(int n) {
            long res[] = new long[n];
            for (int i = 0; i < n; i++)
                res[i] = nextLong();
            return res;
        }
    }

    static FastReader f = new FastReader();
    static BufferedWriter w = new BufferedWriter(new OutputStreamWriter(System.out));

    static boolean isPrime(long n) {
        if (n <= 1)
            return false;
        if (n <= 3)
            return true;
        if (n % 2 == 0 || n % 3 == 0)
            return false;
        for (long i = 5; i * i <= n; i = i + 6)
            if (n % i == 0 || n % (i + 2) == 0)
                return false;
        return true;
    }

    static int lowerBound(int a[], int x) { // x is the target, returns lowerBound. If not found return -1
        int l = -1, r = a.length, flag = 0;
        while (l + 1 < r) {
            int m = (l + r) >>> 1;
            if (x == a[m])
                flag = 1;
            if (a[m] >= x)
                r = m;
            else
                l = m;
        }
        return flag == 0 ? -1 : r;
    }

    static int upperBound(int a[], int x) {// x is the target, returns upperBound. If not found return -1
        int l = -1, r = a.length, flag = 0;
        while (l + 1 < r) {
            int m = (l + r) >>> 1;
            if (a[m] == x)
                flag = 1;
            if (a[m] <= x)
                l = m;
            else
                r = m;
        }
        return flag == 0 ? -1 : (l + 1);
    }

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

    static long lcm(long a, long b) {
        return (a * b) / gcd(a, b);
    }

    static long power(long x, long y) {
        long res = 1;
        while (y > 0) {
            if (y % 2 == 1)
                res = (res * x);
            y >>= 1;
            x = (x * x);
        }
        return res;
    }

    public double binPow(double x, int n) {// binary exponentiation with negative power as well
        if (n == 0)
            return 1.0;
        double binPow = binPow(x, n / 2);
        if (n % 2 == 0) {
            return binPow * binPow;
        } else {
            return n > 0 ? (binPow * binPow * x) : (binPow * binPow / x);
        }
    }

    static long ceil(long x, long y) {
        return (x % y == 0 ? x / y : (x / y + 1));
    }

    /*
     * ===========Modular Operations==================
     */
    static long modPower(long x, long y, long p) {
        long res = 1;
        x = x % p;
        while (y > 0) {
            if (y % 2 == 1)
                res = (res * x) % p;
            y = y >> 1; // y = y/2
            x = (x * x) % p;
        }
        return res;
    }

    static long modInverse(long n, long p) {
        return modPower(n, p - 2, p);
    }

    static long modAdd(long a, long b) {
        return ((a + b + mod) % mod);
    }

    static long modMul(long a, long b) {
        return ((a % mod) * (b % mod)) % mod;
    }

    static long nCrModPFermat(int n, int r) {
        if (r == 0)
            return 1;
        long[] fac = new long[n + 1];
        fac[0] = 1;
        for (int i = 1; i <= n; i++)
            fac[i] = fac[i - 1] * i % mod;
        return (fac[n] * modInverse(fac[r], mod) % mod * modInverse(fac[n - r], mod) % mod) % mod;
    }

    /*
     * ===============================================
     */

    static void ruffleSort(long[] a) {
        int n = a.length;
        Random r = new Random();
        for (int i = 0; i < a.length; i++) {
            int oi = r.nextInt(n);
            long temp = a[i];
            a[i] = a[oi];
            a[oi] = temp;
        }
        Arrays.sort(a);
    }

    static void ruffleSort(int[] a) {
        int n = a.length;
        Random r = new Random();
        for (int i = 0; i < a.length; i++) {
            int oi = r.nextInt(n);
            int temp = a[i];
            a[i] = a[oi];
            a[oi] = temp;
        }
        Arrays.sort(a);
    }

    static String binString32(int n) {
        StringBuilder str = new StringBuilder("");
        String bin = Integer.toBinaryString(n);
        if (bin.length() != 32) {
            for (int k = 0; k < 32 - bin.length(); k++) {
                str.append("0");
            }
            str.append(bin);
        }
        return str.toString();
    }

    /*
     * ====================================Main=================================
     */

    // static int st[], a[];

    // static void buildTree(int treeIndex, int lo, int hi) {
    // if (hi == lo) {
    // st[treeIndex] = a[lo];
    // return;
    // }
    // int mid = (lo + hi) / 2;
    // buildTree(treeIndex * 2 + 1, lo, mid);
    // buildTree(treeIndex * 2 + 2, mid + 1, hi);
    // st[treeIndex] = merge(st[treeIndex * 2 + 1], st[treeIndex * 2 + 2]);
    // }

    // static void update(int treeIndex, int lo, int hi, int arrIndex, int val) {
    // if (hi == lo) {
    // st[treeIndex] = val;
    // a[arrIndex] = val;
    // return;
    // }
    // int mid = (hi + lo) / 2;
    // if (mid < arrIndex) {
    // update(treeIndex * 2 + 2, mid + 1, hi, arrIndex, val);
    // } else {
    // update(treeIndex * 2 + 1, lo, mid, arrIndex, val);
    // }
    // st[treeIndex] = merge(st[treeIndex * 2 + 1], st[treeIndex * 2 + 2]);
    // }

    // static int query(int treeIndex, int lo, int hi, int l, int r) {
    // if (l <= lo && r >= hi) {
    // return st[treeIndex];
    // }
    // if (l > hi || r < lo) {
    // return 0;
    // }
    // int mid = (hi + lo) / 2;
    // return query(treeIndex * 2 + 1, lo, mid, l, Math.min(mid, r));
    // }

    // static int merge(int a, int b) {
    // return a + b;
    // }
    public static void main(String args[]) throws Exception {
        int t = 1;
        // t = f.nextInt();
        int tc = 1;
        while (t-- != 0) {
            int n = f.nextInt(), flag = 0;
            int a[] = new int[n];
            a = f.intArr(n);
            if (n == 1) {
                w.write("-1\n");
                continue;
            }
            ruffleSort(a);
            HashMap<Integer, Integer> hm = new HashMap<>();
            int d = 0;
            for (int i = 1; i < n; i++) {
                d = a[i] - a[i - 1];
                if (hm.get(d) == null) {
                    hm.put(d, 0);
                }
                hm.put(d, hm.get(d) + 1);
            }

            if (hm.size() == 1) {
                TreeSet<Integer> ans = new TreeSet<>();
                ans.add(a[0] - d);
                ans.add(a[n - 1] + d);
                if (d % 2 == 0 && n == 2) {
                    ans.add(a[0] + d / 2);
                }
                w.write(ans.size() + "\n");
                for (int i : ans) {
                    w.write(i + " ");
                }
                w.write("\n");
                flag = 1;
            } else if (hm.size() == 2) {
                int p[] = new int[2];
                int pos = 0;
                for (int i : hm.keySet()) {
                    p[pos] = i;
                    pos++;
                }
                if (Math.min(p[0], p[1]) == 0 || hm.get(Math.max(p[0], p[1])) > 1) {
                    w.write("0\n");
                    flag = 1;
                } else {
                    if (Math.max(p[0], p[1]) % 2 == 0) {
                        for (int i = 1; i < n; i++) {
                            d = a[i] - a[i - 1];
                            // System.out.println((a[i]-d/2)-a[i-1]);
                            if (d == Math.max(p[0], p[1]) && (a[i]-d/2)-a[i-1]==Math.min(p[0],p[1])) {
                                w.write(1 + "\n");
                                w.write((a[i] - d / 2 + "\n"));
                                flag = 1;
                                break;
                            }
                        }
                    }
                }
            }
            if (flag == 0) {
                w.write("0\n");
            }

        }
        w.flush();
    }
}
// 3*3*3*4*2