package main

import . "fmt"
import . "slices"

func main() {
	var n int
	Scan(&n)
	t:=make([]int, n)
	for i:=range t {
		Scan(&t[i])
	}
	ans:=-1
	for d:=0; d<=11; d++ {
		x:=[]int{0,2,4,5,7,9,11}
		for i,v :=range x {
			x[i] = (d+v)%12
		}
		ok := true
		for _, v:=range t {
			ok = ok && Contains(x, v)
		}
		if ok {
			if ans < 0 {
				ans = d
			} else {
				Println(-1)
				return
			}
		}
	}
	Println(ans)
}