#include <cstdio>

#include <cstring>

#include <iostream>

#define sid 200050

#define ri register int

using namespace std;

template <typename re>

inline void upmax(re &a, re b) { if(a < b) a = b; }

int Tt, n, ml, ned;

char s[sid];

int bel[sid], sc[sid];

int flag[];

int sa[sid], rk[sid], cnt[sid], p1[sid], p2[sid], ht[sid];

inline void Suffix() {

    int m = ;

    int *t1 = p1, *t2 = p2;

    for(ri i = ; i <= n; i ++) sc[i] = (s[i] == ) ? ++ m : s[i];

    for(ri i = ; i <= n; i ++) t1[i] = sc[i];

    for(ri i = ; i <= m; i ++) cnt[i] = ;

    for(ri i = ; i <= n; i ++) cnt[t1[i]] ++;

    for(ri i = ; i <= m; i ++) cnt[i] += cnt[i - ];

    for(ri i = n; i >= ; i --) sa[cnt[t1[i]] --] = i;

    for(ri k = ; k <= n; k <<= ) {

        int p = ;

        for(ri i = ; i <= m; i ++) t2[i] = ;

        for(ri i = n - k + ; i <= n; i ++) t2[++ p] = i;

        for(ri i = ; i <= n; i ++) if(sa[i] > k) t2[++ p] = sa[i] - k;

        for(ri i = ; i <= m; i ++) cnt[i] = ;

        for(ri i = ; i <= n; i ++) cnt[t1[t2[i]]] ++;

        for(ri i = ; i <= m; i ++) cnt[i] += cnt[i - ];

        for(ri i = n; i >= ; i --) sa[cnt[t1[t2[i]]] --] = t2[i];

        swap(t1, t2); t1[sa[]] = p = ;

        for(ri i = ; i <= n; i ++)

        t1[sa[i]] = (t2[sa[i]] == t2[sa[i - ]] && t2[sa[i] + k] == t2[sa[i - ] + k]) ? p : ++ p;

        m = p; if(p >= n) break;

    }

    for(ri i = ; i <= n; i ++) rk[sa[i]] = i;

    ri k = , j;

    for(ri i = ; i <= n; i ++) {

        if(k) k --;

        j = sa[rk[i] - ];

        while(sc[j + k] == sc[i + k]) k ++;

        ht[rk[i]] = k;

    }

}

inline bool Check(int htk) {

    int cnt = , tim = ;

    memset(flag, , sizeof(flag));

    for(ri i = ; i <= n; i ++) {

        if(ht[i] < htk) cnt = , ++ tim;

        if(flag[bel[sa[i]]] != tim && bel[sa[i]]) cnt ++, flag[bel[sa[i]]] = tim;

        if(cnt >= ned) return ;

    }

    return ;

}

inline int Binary() {

    int l = , r = ml, ans = -;

    while(l <= r) {

        int mid = (l + r) >> ;

        if(Check(mid)) l = mid + , ans = mid;

        else r = mid - ;

    }

    return ans;

}

inline void Get_Ans(int op) {

    if(op == -) {

        printf("?\n");

        return;

    }

    memset(flag, , sizeof(flag));

    int cnt = , tim = , fag;

    for(ri i = ; i <= n; i ++) {

        if(ht[i] < op) fag = , cnt = , ++ tim;

        if(flag[bel[sa[i]]] != tim && bel[sa[i]]) cnt ++, flag[bel[sa[i]]] = tim;

        if(cnt >= ned && !fag) {

            int k = sa[i];

            for(ri j = ; j <= op; j ++) printf("%c", s[k + j - ]);

            printf("\n");

            cnt = ; fag = ;

        }

    }

}

int main() {

    bool pe = ;

    while(scanf("%d", &Tt) ==  && Tt) {

        if(pe) printf("\n"); pe = ;

        n = ml = ; ned = Tt /  + ;

        memset(s, , sizeof(s));

        memset(bel, , sizeof(bel));

        for(ri i = ; i <= Tt; i ++) {

            scanf("%s", s +  + n);

            int nl = strlen(s +  + n);

            for(ri j = n + ; j <= n + nl; j ++) bel[j] = i;

            upmax(ml, nl); n += nl; s[++ n] = ;

        }

        if(Tt != ) {

            Suffix();

            int ans = Binary();

            Get_Ans(ans);

        }

        else {

            for(ri j = ; j <= n - ; j ++) printf("%c", s[j]);

            printf("\n");

        }

    }

    return ;

}