#include<stdio.h>
int f(int n)
{
	n = (n % 12 + 12) % 12;
	if (n == 0 || n == 2 || n == 4 || n == 5 || n == 7 || n == 9 || n == 11)
		return 0;
	else
		return 1;
}
int main()
{
	int n;
	scanf("%d", &n);
	int i, j;
	int t[16];
	for (i = 0; i < n; i++)
		scanf("%d", &t[i]);
	int cnt = 0, v;
	for (i = 0; i < 12; i++)
	{
		v = 0;
		for (j = 0; j < n; j++)
			v += f(t[j] - i);
		if (v == 0)
			cnt++;
	}
	if (cnt == 1)
	{
		for (i = 0; i < 12; i++)
		{
			v = 0;
			for (j = 0; j < n; j++)
				v += f(t[j] - i);
			if (v == 0)
				printf("%d\n", i);
		}
	}
	else
		printf("-1\n");
	return 0;
}