#include<iostream>
#include<string>
#include<iomanip>
#include<cmath>
#include<vector>
#include<algorithm>
#include<utility>

using namespace std;

#define int long long
#define endl "\n"

constexpr long long INF = (long long)1e18;
constexpr long long MOD = 1'000'000'007; 

struct fast_io {
	fast_io(){
		std::cin.tie(nullptr);
		std::ios::sync_with_stdio(false);
	};
} fio;

signed main(){
	cout<<fixed<<setprecision(10);
	
	int N;
	vector<int> con(12);
	
	cin>>N;
	
	for(int i = 0; i < N; i++){
		int t;
		
		cin>>t;
		
		con[t]++;
	}
	
	int c = 0, x = -1;
	
	for(int i = 0; i < 12; i++){
		if(con[(i+1)%12]) continue;
		if(con[(i+3)%12]) continue;
		if(con[(i+6)%12]) continue;
		if(con[(i+8)%12]) continue;
		if(con[(i+10)%12]) continue;
		c++;
		x = i;
		// cout<<i<<endl;
	}
	
	if(c == 1) cout<<x<<endl;
	else cout<<-1<<endl;
	
	return 0;
}