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)
		S[i] = t
	}


	encount_str := make(map[string]int)
	for _, v := range S {
		for i := 0; i < 26; i++ {
			s := v + string(rune('a' + i))
			r := []rune(s)
			sort.Slice(r, func(i, j int) bool { return r[i] < r[j] })
			s = string(r)
			encount_str[s]++
		}
	}
	
	for k, v := range encount_str {
		if v == 1 {
			fmt.Println(k)
			return
		}
	}
	fmt.Println(-1)
}