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("!
")
#define INF 10000
#define MAXN 5010
#define MAX(a,b) a>b?a:b
#define blank pf("
")
#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[100005];

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

    }
}

#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("!
")
#define INF 10000
#define MAXN 5010
#define MAX(a,b) a>b?a:b
#define blank pf("
")
#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[100005];
map<int,int> mp;


int main()
{
    int i,j;
    while(sf("%d",&n)==1)
    {
        mp.clear();
        for(i=0;i<n;i++)
        {
            sf("%d",&a[i]);
            mp[a[i]]++;
        }
        map<int,int>::iterator it;
        int res[3],cnt=0,ok=0;
        for(it=mp.begin();it!=mp.end();it++)
        {
            if(it->second)
            {
                res[cnt++] = it->first;
                it->second--;
                if(cnt==3)
                {
                    pf("%d %d %d
",res[2],res[1],res[0]);
                    cnt=0;
                    ok=1;
                    it = mp.begin();
                }
            }
        }
        if(!ok) pf("0
");
    }
}