#include <stdio.h>
#include <bits/stdc++.h>

using namespace std;

#define ll long long
#define ld long double
#define pb push_back
#define mp make_pair
#define pii pair<int, int>
#define pll pair<ll, ll>
#define pdd pair<ld, ld>
#define all(x) (x).begin(), (x).end()
#define fi first
#define se second

const int kMaxN = 105;
const int b1 = 31;
const int b2 = 37;
const int mod1 = 100003;
const int mod2 = 666013;

int n;
int sz[kMaxN];
int H1[kMaxN];
int H2[kMaxN];
bool dp[kMaxN];
char s[kMaxN][kMaxN];
map<pii, bool> m;

int main() {
  cin.sync_with_stdio(false);

  cin >> n;

  for (int i = 1; i <= n; i++) {
    cin >> (s[i] + 1);
    sz[i] = strlen(s[i] + 1);
    int h1 = 0;
    int h2 = 0;
    for (int j = 1; j <= sz[i]; j++) {
      h1 = (h1 * b1 + s[i][j] - 'a' + 1) % mod1;
      h2 = (h2 * b2 + s[i][j] - 'a' + 1) % mod2;
    }
    H1[i] = h1;
    H2[i] = h2;
    m[mp(h1, h2)] = 1;
  }

  int anssz = -1;
  int ans = 0;
  for (int w = 1; w <= n; w++) {
    m.erase(m.find(mp(H1[w], H2[w])));
    for (int i = 1; i <= sz[w]; i++) {
      dp[i] = 0;
    }

    dp[0] = 1;
    for (int i = 1; i <= sz[w]; i++) {
      if (dp[i - 1]) {
        int h1 = 0;
        int h2 = 0;
        for (int j = i; j <= sz[w]; j++) {
          h1 = (h1 * b1 + s[w][j] - 'a' + 1) % mod1;
          h2 = (h2 * b2 + s[w][j] - 'a' + 1) % mod2;
          if (m.count(mp(h1, h2))) {
            dp[j] = 1;
          }
        }
      }
    }

    if (dp[sz[w]]) {
      if (sz[w] > anssz) {
        anssz = sz[w];
        ans = w;
      }
    }

    m[mp(H1[w], H2[w])] = 1;
  }

  if (anssz != -1) {
    cout << (s[ans] + 1) << '\n';
  } else {
    cout << -1 << '\n';
  }

  return 0;
}