package main

import (
	"fmt"
	"sort"
)

func sum[T int | float64](nums []T) T {
	var sum T
	for _, n := range nums {
		sum += n
	}
	return sum
}

func reverse(s string) string {
	var r []rune = []rune(s)
	for i, j := 0, len(r)-1; i < j; i, j = i+1, j-1 {
		r[i], r[j] = r[j], r[i]
	}
	return string(r)
}

func main() {
	var N int
	fmt.Scan(&N)
	var S []string = make([]string, N)

	for i := 0; i < N; i++ {
		var t string
		fmt.Scan(&t)
		r := []rune(t)
		sort.Slice(r, func(i, j int) bool { return r[i] < r[j] })
		S[i] = string(r)
	}

	sort.Slice(S, func(i, j int) bool { return S[i] < S[j] })

	var not_ans []bool = make([]bool, N)
	for i := 0; i < N-1; i++ {
		if S[i] == S[i+1] {
			not_ans[i] = true
			not_ans[i+1] = true
		}
	}

	for i, v := range not_ans {
		if !v {
			fmt.Println(S[i] + "a")
			return
		}
	}
	fmt.Println(-1)
}