#include <bits/stdc++.h>
#define For(i, a, b) for(int (i)=(int)(a); (i)<(int)(b); ++(i))
#define rFor(i, a, b) for(int (i)=(int)(a)-1; (i)>=(int)(b); --(i))
#define rep(i, n) For((i), 0, (n))
#define rrep(i, n) rFor((i), (n), 0)
#define fi first
#define se second
using namespace std;
typedef long long lint;
typedef unsigned long long ulint;
typedef pair<int, int> pii;
typedef pair<lint, lint> pll;
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(a>b){a=b; return true;} return false;}
template<class T> T div_floor(T a, T b){
    if(b < 0) a *= -1, b *= -1;
    return a>=0 ? a/b : (a+1)/b-1;
}
template<class T> T div_ceil(T a, T b){
    if(b < 0) a *= -1, b *= -1;
    return a>0 ? (a-1)/b+1 : a/b;
}

constexpr lint mod = 1e9+7;
constexpr lint INF = mod * mod;
constexpr int MAX = 200010;

int a[] = {0, 2, 4, 5, 7, 9, 11};
int n, t[15];
vector<int> ans;

void dfs(int cur, int d){
    if(cur == n){
        ans.push_back(d);
        return;
    }
    rep(i, 7){
        int tmp = (t[cur]-a[i]+12)%12;
        if(d >= 0 && d != tmp) continue;
        dfs(cur+1, tmp);
    }
}

int main(){
    scanf("%d", &n);
    rep(i, n) scanf("%d", &t[i]);
    dfs(0, -1);
    sort(ans.begin(), ans.end());
    ans.erase(unique(ans.begin(), ans.end()), ans.end());
    if(ans.empty() || ans.size() >= 2) puts("-1");
    else printf("%d\n", ans[0]);
}