N=int(input()) T=list(map(int,input().split())) X=[0,2,4,5,7,9,11] l=[] for i in T: sw=0 for j in T: if (j-i)%12 in X: pass else: sw=1 break if sw==0: l.append(i) if len(l)==1: print(l[0]) else: print(-1)