#include <iostream>

#include <cstring>

#include <cstdio>

using namespace std;  

const int MAX = 100500;

const int nMAX = 105;

const int mMAX = 1005;  

int strnum;

char str[nMAX][mMAX];

int source[MAX];

int sa[MAX], rk[MAX], height[MAX];

int wa[MAX], wb[MAX], wv[MAX], wd[MAX];

bool vis[nMAX];

int id[MAX];

int anslen, anspos[mMAX], ansnum;

  const int MAXN=200000+100;

void radix(int *str,int *a,int *b,int n,int m)

{

 static int count[MAXN];

 memset(count,0,sizeof(count));

 for(int i=0;i<n;++i)++count[str[a[i]]];

 for(int i=1;i<=m;++i)count[i]+=count[i-1];

 for(int i=n-1;i>=0;--i)b[--count[str[a[i]]]]=a[i];

}

void sorted_suffix_array(int *str,int *sa,int n,int m)

{

 static int rank[MAXN],a[MAXN],b[MAXN];

 for(int i=0;i<n;++i)rank[i]=i;

 radix(str,rank,sa,n,m);

 rank[sa[0]]=0;

 for(int i=1;i<n;++i)rank[sa[i]]=rank[sa[i-1]]+(str[sa[i]]!=str[sa[i-1]]);

 for(int i=0;(1<<i) <n;++i)

 	{

 	 for(int j=0;j<n;++j)

 	 	{

 	 	 a[j]=rank[j]+1;

 	 	 b[j]=j+(1<<i)>=n? 0:rank[j+(1<<i)]+1;

 	 	 sa[j]=j;

		}

	 radix(b,sa,rank,n,n);

	 radix(a,rank,sa,n,n);

	 rank[sa[0]]=0;

	 for(int j=1;j<n;++j)

	 	{

	 	 rank[sa[j]]=rank[sa[j-1]]+(a[sa[j-1]]!=a[sa[j]]||b[sa[j-1]]!=b[sa[j]]);

		}

	}

}

void calc_height(int *str,int *sa,int *h,int n)

{

 static int Rank[MAXN];

 int k=0;

 h[0]=0;

 for(int i=0;i<n;++i)Rank[sa[i]]=i;

 for(int i=0;i<n;++i)

 	{

 	 k= k==0?0:k-1;

 	 if(Rank[i]!=0)

 	   while(str[i+k]==str[sa[Rank[i]-1]+k])++k;

 	 h[Rank[i]]=k;

	}

}

bool solve(int beg, int end)

{

    int tot = 0;

    int t = strnum >> 1;

    for (int i = 0; i < strnum; ++i) vis[i] = false;

    for (int i = beg; i <= end; ++i)

    {

        if (!vis[id[sa[i]]])

        {

            vis[id[sa[i]]] = true;

            ++tot;

        }

        if (tot > t) return true;

    }

    return false;

}  

bool group(int len, int n)

{

    bool res = false;

    int beg, end;

    beg = end = 0;

    for (int i = 1; i < n; ++i)

    {

        if (height[i] >= len) ++end;

        else

        {

            if (solve(beg, end))

            {

                if (!res) ansnum = 0;

                res = true;

                anspos[ansnum++] = sa[beg];

            }

            beg = end = i;

        }

    }

    if (beg < end)

    {

        if (solve(beg, end))

        {

            if (!res) ansnum = 0;

            res = true;

            anspos[ansnum++] = sa[beg];

        }

    }

    return res;

}  

int main()

{ // freopen("t.txt","r",stdin);

 bool flg=false;

    while (scanf("%d", &strnum) && strnum != 0)

    {   if(flg)printf("\n");

        for (int i = 0; i < strnum; ++i) scanf("%s", str[i]);

        int n = 0, low = 1, high = 0, mid;

        for (int i = 0; i < strnum; ++i)

        {

            int j;

            for (j = 0; str[i][j] != 0; ++j)

            {

                id[n] = i;

                source[n++] = str[i][j] - 'a' + 100;

            }

            if (j > high) high = j;

            id[n] = i;

            source[n++] = i;

        }

        sorted_suffix_array(source,sa,n,126);

        calc_height(source,sa,height,n);

        //suffix(source, n, 126);

        //calheight(source, n - 1);

        anslen = 0;

        while (low <= high)

        {

            mid = (low + high) >> 1;

            if (group(mid, n))

            {

                anslen = mid;

                low = mid + 1;

            }

            else high = mid - 1;

        }

        if (anslen == 0) printf("?\n");

        else

        {

            for (int i = 0; i < ansnum; ++i)

            {

                for (int j = 0; j < anslen; ++j)

                {

                    printf("%c", source[anspos[i] + j] - 100 + 'a');

                }

                printf("\n");

            }

        }

        //printf("\n");

		flg=true;

    }

    return 0;

}