import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner();
        int n = sc.nextInt();
        int[] values = new int[n];
        for (int i = 0; i < n; i++) {
            values[i] = sc.nextInt();
        }
        int[] bases = new int[]{0, 2, 4, 5, 7, 9, 11};
        HashSet<Integer> enables = new HashSet<>();
        for (int x : bases) {
            enables.add(x);
        }
        int ans = -1;
        for (int i = 0; i < 12; i++) {
            boolean can = true;
            for (int j = 0; j < n && can; j++) {
                can = enables.contains((values[j] - i + 12) % 12);
            }
            if (can) {
                if (ans == -1) {
                    ans = i;
                } else {
                    System.out.println(-1);
                    return;
                }
            }
        }
        System.out.println(ans);
    }
}
    
class Scanner {
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    StringTokenizer st = new StringTokenizer("");
    StringBuilder sb = new StringBuilder();
    
    public Scanner() throws Exception {
        
    }
    
    public int nextInt() throws Exception {
        return Integer.parseInt(next());
    }
    
    public long nextLong() throws Exception {
        return Long.parseLong(next());
    }
    
    public double nextDouble() throws Exception {
        return Double.parseDouble(next());
    }
    
    public String next() throws Exception {
        while (!st.hasMoreTokens()) {
            st = new StringTokenizer(br.readLine());
        }
        return st.nextToken();
    }
    
}