#include <bits/stdc++.h>
using namespace std;

#define lli long long int
#define REP(i,s,n) for(int i=s;i<n;i++)
#define NUM 2520
#define INF (1LL<<50)
#define DEBUG 0
#define mp(a,b) make_pair(a,b)
#define SORT(V) sort(V.begin(),V.end())
#define PI (3.141592653589794)
#define TO_STRING(VariableName) # VariableName
#define LOG(x) if(DEBUG)cout<<TO_STRING(x)<<"="<<x<<" ";
#define LOGE if(DEBUG)cout<<endl;

template<class T>bool chmax(T & a, const T & b) { if (a < b) { a = b; return 1; } return 0; }
template<class T>bool chmin(T& a, const T& b) { if (b < a) { a = b; return 1; } return 0; }

int main(){

	lli base[7]={0,2,4,5,7,9,11};

	lli n;
	cin>>n;

	vector<lli> T(n);
	REP(i,0,n)cin>>T[i];

	set<lli> ans;
	for(lli i=0;i<12;i++){
		lli nowBase[7];
		REP(j,0,7)nowBase[j] = (base[j]+i)%12;

		bool ok = true;
		REP(j,0,n){
			bool okk = false;
			REP(k,0,7){
				if(T[j] == nowBase[k]){
					okk = true;
					break;
				}
			}
			if(!okk){
				ok = false;
				break;
			}
		}
		if(ok){
			ans.insert(i);
		}
	}
	if(ans.size()!=1){
		cout<<-1<<endl;
		if(DEBUG){
			for(auto e:ans){
				LOG(e);
			}
		}
	}
	else{
		cout<<(*ans.begin())<<endl;
	}

    return 0;
}