#include <bits/stdc++.h>
#define ll long long
using namespace std;
ll n;
string s[110];
int a[110][110];

bool cmp(string a, string b)
{
    return (a.size() < b.size());
}

void poz(int x, int y)
{
    string c = s[x], S = s[y];
    int i = 0, j = 0;
    while (i < S.size())
    {
        j = 0;bool u = 0;
        while (S[i] == c[j] && j < c.size() && i < S.size()) i++, j++, u = 1;
        if (j == c.size()) a[x][y] += c.size();
        if (u) continue;
        else i++;
    }
}

int main(){
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cin >> n;
    for (int i = 1; i <= n; i++)
        cin >> s[i];
    sort(s+1, s+n+1,cmp);
    for (int i = 1; i <= n; i++)
    {
        for (int j = i + 1; j <= n; j++)
            poz(i, j);
    }
    for (int i = n; i; i--)
    {
        int sum = 0;
        for (int j = 1; j < i; j++) sum += a[j][i];
        if (sum >= s[i].size()) return cout << s[i], 0;
    }
    cout << -1;
}