SZU4

#include <iostream>
#include <string>
#include <cstring>
#include <cstdlib>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <stack>
#include <queue>
#include <cctype>
#include <vector>
#include <iterator>
#include <set>
#include <map>
#include <sstream>
using namespace std;

#define mem(a,b) memset(a,b,sizeof(a))
#define pf printf
#define sf scanf
#define spf sprintf
#define pb push_back
#define debug printf("!\n")
#define INF 10000
#define MAXN 5010
#define MAX(a,b) a>b?a:b
#define blank pf("\n")
#define LL long long
#define ALL(x) x.begin(),x.end()
#define INS(x) inserter(x,x.begin())
#define pqueue priority_queue

int n;
int mp[];

int main()
{
    int i,j;
    while(sf("%d",&n)==)
    {
        mem(mp,);
        for(i=;i<n;i++)
        {
            sf("%d",&mp[i]);
        }
        sort(mp,mp+n);
        if(n==)
        {
            pf("-1\n");
            continue;
        }
        if(n==)
        {
            int d = mp[]-mp[];
            if(d==)
            {
                pf("1\n%d\n",mp[]);
            }
            else if(d%==)
            {
                pf("3\n%d %d %d\n",mp[]-d,mp[]+d/,mp[]+d);
            }
            else
            {
                pf("2\n%d %d\n",mp[]-d,mp[]+d);
            }
        }
        if(n==)
        {
            int d = mp[]-mp[];
            int d2 = mp[]-mp[];
            if(d==d2)
            {
                pf("2\n%d %d\n",mp[]-d,mp[]+d);
            }
            else if(d==d2*)
                pf("1\n%d\n",mp[]+d2);
            else if(d*==d2)
                pf("1\n%d\n",mp[]+d);
            else
                pf("0\n");
        }
        if(n>)
        {
            int cnt=,cnt2=,find=,d2,ok=,x;
            int d = mp[]-mp[];
            for(i=;i<n-;i++)
            {
                int tmp = mp[i+]-mp[i];
                if(d==tmp) cnt++;
                else
                {
                    if(!find)
                    {
                        d2=tmp;
                        find=;
                        x = i;
                    }
                    if(tmp==d2)
                        cnt2++;
                    else
                    {
                        ok=;break;
                    }
                }
            }
            if(!ok)
            {
                pf("0\n");
                continue;
            }
            if(cnt==n-)
            {
                pf("2\n%d %d\n",mp[]-d,mp[n-]+d);
            }
            else if(cnt==n-)
            {
                if(d2==d*)
                {
                    pf("1\n%d\n",mp[x]+d);
                }
                else
                    pf("0\n");
            }
            else
            {
                if(d==d2*)
                {
                    pf("1\n%d\n",mp[]+d2);
                }
                else
                    pf("0\n");
            }
        }

    }
}

#include <iostream>
#include <string>
#include <cstring>
#include <cstdlib>
#include <cstdio>
#include <cmath>
#include <algorithm>
#include <stack>
#include <queue>
#include <cctype>
#include <vector>
#include <iterator>
#include <set>
#include <map>
#include <sstream>
using namespace std;

#define mem(a,b) memset(a,b,sizeof(a))
#define pf printf
#define sf scanf
#define spf sprintf
#define pb push_back
#define debug printf("!\n")
#define INF 10000
#define MAXN 5010
#define MAX(a,b) a>b?a:b
#define blank pf("\n")
#define LL long long
#define ALL(x) x.begin(),x.end()
#define INS(x) inserter(x,x.begin())
#define pqueue priority_queue

int n;
int a[];
map<int,int> mp;

int main()
{
    int i,j;
    while(sf("%d",&n)==)
    {
        mp.clear();
        for(i=;i<n;i++)
        {
            sf("%d",&a[i]);
            mp[a[i]]++;
        }
        map<int,int>::iterator it;
        int res[],cnt=,ok=;
        for(it=mp.begin();it!=mp.end();it++)
        {
            if(it->second)
            {
                res[cnt++] = it->first;
                it->second--;
                if(cnt==)
                {
                    pf("%d %d %d\n",res[],res[],res[]);
                    cnt=;
                    ok=;
                    it = mp.begin();
                }
            }
        }
        if(!ok) pf("0\n");
    }
}