package main

import "fmt"

func main() {
	var N int
	fmt.Scan(&N)

	T := make([]int, N)

	for i := 0; i < N; i++ {
		fmt.Scan(&T[i])
	}

	var flag int
	ans := make([]int, 0)
	for D := 0; D < 12; D++ {
		flag = 0
		for i := 0; i < N; i++ {
			if T[i] == D%12 || T[i] == (D+2)%12 || T[i] == (D+4)%12 || T[i] == (D+5)%12 || T[i] == (D+7)%12 || T[i] == (D+9)%12 || T[i] == (D+11)%12 {
				continue
			} else {
				flag++
			}
		}

		if flag == 0 {
			ans = append(ans, D)
		}
	}

	if len(ans) == 0 {
		fmt.Println("-1")
	} else if len(ans) == 1 {
		fmt.Println(ans[0])
	} else {
		fmt.Println("-1")
	}
}