#!/usr/bin/env python3
from typing import *

def solve(n: int, t: List[int]) -> int:
    ans = []
    for d in range(12):
        for t_i in t:
            if t_i not in list(map(lambda x: (d + x) % 12, [0, 2, 4, 5, 7, 9, 11])):
                break
        else:
            ans.append(d)
    if len(ans) == 1:
        return ans[0]
    else:
        return -1

# generated by online-judge-template-generator v4.4.0 (https://github.com/kmyk/online-judge-template-generator)
def main():
    import sys
    tokens = iter(sys.stdin.read().split())
    N = int(next(tokens))
    T = [None for _ in range(N)]
    for i in range(N):
        T[i] = int(next(tokens))
    assert next(tokens, None) is None
    a = solve(N, T)
    print(a)

if __name__ == '__main__':
    main()