import java.io.InputStreamReader;
import java.io.IOException;
import java.io.BufferedReader;

public class Main{
    public static void main(String args[]) throws IOException{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        
        String line = br.readLine();
        if(line.charAt(line.length()-1) == ' ') System.exit(1);
        int n = Integer.parseInt(line);
        if(!(1 <= n && n <= 12)) System.exit(1);

        line = br.readLine();
        if(line.charAt(line.length()-1) == ' ') System.exit(1);
        String[] tmp = line.split(" ");
        if(tmp.length != n) System.exit(1);

        int[] arr = new int[n];
        for(int i = 0; i < n; i++)  arr[i] = Integer.parseInt(tmp[i]);

        for(int i = 1; i < n; i++){
            if(!(arr[i-1] < arr[i]))   System.exit(1);
        }

        boolean[] x = new boolean[12];
        for(int i = 0; i < n; i++){
            if(!(0 <= arr[i] && arr[i] < 12))   System.exit(1);
            x[arr[i]] = true;
        }

        int[] add = {0, 2, 4, 5, 7, 9, 11};
        int ans = -1;
        for(int i = 0; i < 12; i++){
            int count = 0;
            for(int j : add){
                if(x[(i+j)%12]) count++;
            }
            if(count != n)  continue;
            if(ans != -1){
                ans = -1;
                break;
            }
            ans = i;
        }
        System.out.println(ans);
    }
}