import java.io.BufferedReader
import java.io.InputStream
import java.io.InputStreamReader
import java.io.PrintWriter
import java.util.*

fun PrintWriter.solve(sc: FastScanner) {
    val n = sc.nextInt()
    val t = Array(n) { sc.nextInt() }
    val lst = mutableListOf<Int>()
    val a = mutableListOf(0, 2, 4, 5, 7, 9, 11)
    for (d in 0 until 12) {
        var ok = true
        for (i in 0 until n) {
            var ok2 = false
            for (j in 0 until 7) {
                if (t[i] % 12 == (d + a[j]) % 12) {
                    ok2 = true
                    break
                }
            }
            if (!ok2) {
                ok = false
                break
            }
        }
        if (ok) {
            lst.add(d)
        }
    }
    println(if (lst.count() == 1) lst[0] else -1)
}

fun main() {
    val writer = PrintWriter(System.out, false)
    writer.solve(FastScanner(System.`in`))
    writer.flush()
}

class FastScanner(s: InputStream) {
    private var st = StringTokenizer("")
    private val br = BufferedReader(InputStreamReader(s))

    fun next(): String {
        while (!st.hasMoreTokens()) st = StringTokenizer(br.readLine())

        return st.nextToken()
    }

    fun nextInt() = next().toInt()
    fun nextLong() = next().toLong()
    fun nextLine() = br.readLine()
    fun nextDouble() = next().toDouble()
    fun ready() = br.ready()
}