import java.util.*;
import java.io.*;
 
public class Main {
    public static void main(String[] args) throws Exception {
        FastScanner sc = new FastScanner(System.in);
        int n = sc.nextInt();
        int[] a = sc.nextIntArray(n);
        int cnt = 12;
        int[] h = {0,2,4,5,7,9,11};
        ArrayList<ArrayList<Integer>> map = new ArrayList<>();
        for(int i = 0; i < 12; i++){
            ArrayList<Integer> tmp = new ArrayList<Integer>(); 
            for(int hv : h){
                tmp.add((i+hv)%12);
            }
            map.add(tmp);
        }
        boolean[] isNot = new boolean[12];
        for(int i = 0; i < n; i++){
            for(int j = 0; j < 12; j++){
                if(!isNot[j] && !map.get(j).contains(a[i])){
                    isNot[j] = true;
                    cnt--;
                }
            }
        }
        if(cnt != 1){
            System.out.println(-1);
        }else{
            for(int i = 0; i < 12; i++){
                if(!isNot[i]){
                    System.out.println(i);
                    break;
                }
            }
        }
    }
}

class FastScanner {
    private BufferedReader reader = null;
    private StringTokenizer tokenizer = null;
    public FastScanner(InputStream in) {
        reader = new BufferedReader(new InputStreamReader(in));
        tokenizer = null;
    }

    public String next() {
        if (tokenizer == null || !tokenizer.hasMoreTokens()) {
            try {
                tokenizer = new StringTokenizer(reader.readLine());
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
        return tokenizer.nextToken();
    }

    public String nextLine() {
        if (tokenizer == null || !tokenizer.hasMoreTokens()) {
            try {
                return reader.readLine();
            } catch (IOException e) {
                throw new RuntimeException(e);
            }
        }
        return tokenizer.nextToken("\n");
    }

    public long nextLong() {
        return Long.parseLong(next());
    }

    public int nextInt() {
        return Integer.parseInt(next());
    }

    public double nextDouble() {
         return Double.parseDouble(next());
    }
    
    public String[] nextArray(int n) {
        String[] a = new String[n];
        for (int i = 0; i < n; i++)
            a[i] = next();
        return a;
    }

    public int[] nextIntArray(int n) {
        int[] a = new int[n];
        for (int i = 0; i < n; i++)
            a[i] = nextInt();
        return a;
    }

    public long[] nextLongArray(int n) {
        long[] a = new long[n];
        for (int i = 0; i < n; i++)
            a[i] = nextLong();
        return a;
    } 
}