package main

import . "fmt"
import . "slices"

func main() {
	var n int
	Scan(&n)
	m := map[string]int{}
	for i := 0; i < n; i++ {
		var s string
		Scan(&s)
		for c := byte('a'); c <= 'z'; c++ {
			b := append([]byte(s), c)
			Sort(b)
			m[string(b)]++
		}
	}
	for k, cnt := range m {
		if cnt == 1 {
			Println(k)
			return
		}
	}
	Println(-1)
}