public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        PrintWriter out = new PrintWriter(System.out);
        
        int N = in.nextInt();
        long[] cards = new long[N];
        
        for (int i = 0; i < N; i++)
            cards[i] = in.nextLong();
        
        if (N < 2) {
            out.println(-1);
        }
        else {
            Arrays.sort(cards);                        
            
            ArrayList<Long> res1 = new ArrayList<Long>();;          
            long dif = cards[1] - cards[0];
            
            if (dif > 1 && dif % 2 == 0) {
                long incr = dif / 2;
                cards[0] += incr;
                ArrayList<Long> tmp = progression(cards, dif/2, false);
                
                if (tmp.size() > 0) res1.add(cards[0]);
                
                cards[0] -= incr;
            }
            
            ArrayList<Long> res2 = progression(cards, dif, true);
            long[] res = new long[res1.size() + res2.size()];
            int i = 0;
            for (long l: res1) res[i++] = l;                
            for (long l: res2) res[i++] = l;
            
            
            Arrays.sort(res);
            out.println(res.length);
            for (long l: res) out.print(l + " ");
        }
        
        
        out.flush();
    }