#include <cstdio>
#include <cstring>
#include <algorithm>

#define MA 666013LL
#define MB 10009LL
#define MC 2017LL
#define MD 20173333LL

using namespace std;

char str[128][128];
bool ap[128][128];
long long pa[128], pb[128], pc[128], pd[128], ha[128][128], hb[128][128], hc[128][128], hd[128][128];
int len[128];

int main ()
{
   // freopen ("file.in", "r", stdin);

    int n;
    scanf ("%d\n", &n);

    pa[0] = pb[0] = pc[0] = pd[0] = 1LL;
    for (int i = 1; i <= 105; ++i)
        pa[i] = 101LL * pa[i - 1] % MA,
        pb[i] = 101LL * pb[i - 1] % MB,
        pc[i] = 101LL * pc[i - 1] % MC,
        pd[i] = 101LL * pd[i - 1] % MD;

    for (int i = 1; i <= n; ++i)
    {
        gets (str[i] + 1);
        int m = len[i] = strlen (str[i] + 1);

        for (int j = 1; j <= m; ++j)
        {
            ha[i][j] = (ha[i][j - 1] + pa[j - 1] * str[i][j]) % MA;
            hb[i][j] = (hb[i][j - 1] + pb[j - 1] * str[i][j]) % MB;
            hc[i][j] = (hc[i][j - 1] + pc[j - 1] * str[i][j]) % MC;
            hd[i][j] = (hd[i][j - 1] + pd[j - 1] * str[i][j]) % MD;
        }
    }

    int ma = -1, poz;
    for (int i = 1; i <= n; ++i)
    {
        ap[i][0] = true;
        int m = strlen (str[i] + 1);

        for (int j = 1; j <= m; ++j)
        {
            if (!ap[i][j - 1]) continue;

            for (int h = 1; h <= n; ++h)
            {
                if (i == h || j + len[h] - 1 > m) continue;

                int mum = len[h];

                long long na = ha[i][j + mum - 1] - ha[i][j - 1];
                if (na < 0) na += MA;

                long long va = (ha[h][mum] * pa[j - 1]) % MA;

                long long nb = hb[i][j + mum - 1] - hb[i][j - 1];
                if (nb < 0) nb += MB;

                long long vb = (hb[h][mum] * pb[j - 1]) % MB;

                long long nc = hc[i][j + mum - 1] - hc[i][j - 1];
                if (nc < 0) nc += MC;

                long long vc = (hc[h][mum] * pc[j - 1]) % MC;

                long long nd = hd[i][j + mum - 1] - hd[i][j - 1];
                if (nd < 0) nd += MD;

                long long vd = (hd[h][mum] * pd[j - 1]) % MD;

                if (na == va && nb == vb && nc == vc && nd == vd)
                    ap[i][j + mum - 1] = true;
            }
        }

        if (ap[i][m] && m > ma)
            ma = m, poz = i;
    }

    if (ma == -1) printf ("-1\n");
    else puts (str[poz] + 1);

    return 0;
}