import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }

string COLOR(string s = "") { return "\x1b[" ~ s ~ "m"; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


enum A = 26;

void main() {
  try {
    for (; ; ) {
      const N = readInt;
      auto S = new string[N];
      foreach (i; 0 .. N) {
        S[i] = readToken;
      }
      
      auto f = new int[][](N, A);
      foreach (i; 0 .. N) {
        foreach (s; S[i]) {
          ++f[i][s - 'a'];
        }
      }
      f.sort;
      for (int i = 0, j = 0; i < N; i = j) {
        for (; j < N && f[i] == f[j]; ++j) {}
        if (j - i == 1) {
          int am = 0;
          foreach (a; 0 .. A) if (f[i][am] < f[i][a]) am = a;
          foreach (a; 0 .. A) foreach (_; 0 .. f[i][a]) write(cast(char)('a' + a));
          writeln(cast(char)('a' + am));
          goto found;
        }
      }
      
      writeln(-1);
     found:
    }
  } catch (EOFException e) {
  }
}