#include <bits/stdc++.h>
using namespace std;
#define modulo 1000000007
#define mod(mod_x) ((((long long)mod_x+modulo))%modulo)
#define Inf 1000000000

int main(){
	
	int N;
	cin>>N;
	
	vector<int> T(N);
	for(int i=0;i<N;i++)cin>>T[i];
	
	vector<int> ans;
	vector<int> t = {0,2,4,5,7,9,11};
	for(int i=0;i<12;i++){
		bool f = true;
		for(int j=0;j<T.size();j++){
			if(!binary_search(t.begin(),t.end(),(T[j]-i+12)%12)){
				f=false;
				break;
			}
		}
		if(f)ans.push_back(i);
	}
	
	if(ans.size()!=1)cout<<-1<<endl;
	else cout<<ans[0]<<endl;
	
	return 0;
}