#include<bits/stdc++.h> using namespace std; #define ALL(x) x.begin(),x.end() #define rep(i,n) for(int i=0;i<(n);i++) #define debug(v) cout<<#v<<":";for(auto x:v){cout<<x<<' ';}cout<<endl; #define mod 1000000007 using ll=long long; const int INF=1000000000; const ll LINF=1001002003004005006ll; int dx[]={1,0,-1,0}; int dy[]={0,1,0,-1}; // ll gcd(ll a,ll b){return b?gcd(b,a%b):a;} template<class T>bool chmax(T &a,const T &b){if(a<b){a=b;return true;}return false;} template<class T>bool chmin(T &a,const T &b){if(b<a){a=b;return true;}return false;} struct IOSetup{ IOSetup(){ cin.tie(0); ios::sync_with_stdio(0); cout<<fixed<<setprecision(12); } } iosetup; template<typename T1,typename T2> ostream &operator<<(ostream &os,const pair<T1,T2>&p){ os<<p.first<<" "<<p.second; return os; } template<typename T> ostream &operator<<(ostream &os,const vector<T>&v){ for(int i=0;i<(int)v.size();i++) os<<v[i]<<(i+1==v.size()?"":" "); return os; } signed main(){ int n;cin>>n; int t[n]; rep(i,n) cin>>t[i]; auto check=[&](int d){ rep(i,n){ if(t[i]==d or t[i]==((d+2)%12) or t[i]==((d+4)%12) or t[i]==((d+5)%12) or t[i]==((d+7)%12) or t[i]==((d+9)%12) or t[i]==((d+11))%12) continue; return false; } return true; }; vector<int> ans; for(int i=0;i<12;i++){ if(check(i)) ans.push_back(i); } if(ans.size()==1) cout<<ans[0]<<endl; else cout<<-1<<endl; return 0; }