#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;  
}