#include<iostream>
#include<vector>
#include<cassert>
#define rep(i,n) for(i=0;i<(int)(n);i++)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;

int n;

int main(){
    int i,j,k,ans=-1;
    scanf("%d",&n);
    vector<int> t(n);
    rep(i,n)scanf("%d",&t[i]);
    const int dt[]={0,2,4,5,7,9,11},MAX_D=7;
    rep(i,12){
        rep(j,n){
            rep(k,MAX_D)if((i+dt[k])%12==t[j])break;
            if(k==MAX_D)break;
        }
        if(j==n){
            if(ans>=0){
                puts("-1");
                return 0;
            }
            ans=i;
        }
    }
    printf("%d\n",ans);
    return 0;
}