#include <iostream>
using namespace std;
const int N = 1e4 + 5;
int n, m, l;
string st, res;
string s[N];
int main() {
cin >> n;
for (int i = 1;i <= n;i++) {
cin >> s[i];
}
for (int i = 1;i < n;i++) {
for (int j = i+1;j <= n;j++) {
st = s[i].substr(0, m);
if (st != s[j].substr(0, m)) continue;
l = min(s[i].length(), s[j].length());
for (int k = m;k < l;k++) {
if (s[i][k] == s[j][k]) {
st += s[i][k];
} else {
break;
}
}
if (st.length() == m) {
if (res > st) res = st;
} else if (st.length() > m) {
res = st;
m = st.length();
}
}
}
cout << res;
}
