#include <bits/stdc++.h>
using namespace std;
const int dy[] = {-1, 0, 1, 0}, dx[] = {0, 1, 0, -1};
const double EPS = 1e-8;
const double PI = acos(-1.0);
int popcount(int n) { return __builtin_popcount(n); }
int popcount(long long n) { return __builtin_popcountll(n); }
template <class T>
int SIZE(T a) {
  return a.size();
}
template <class T>
string IntToString(T num) {
  string res;
  stringstream ss;
  ss << num;
  return ss.str();
}
template <class T>
T StringToInt(string str) {
  T res = 0;
  for (int i = 0; i < SIZE(str); i++) res = (res * 10 + str[i] - '0');
  return res;
}
template <class T>
T gcd(T a, T b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
template <class T>
T lcm(T a, T b) {
  return a / gcd(a, b) * b;
}
template <class T>
void PrintSeq(T &a, int sz) {
  for (int i = 0; i < sz; i++) {
    cout << a[i];
    if (sz == i + 1)
      cout << endl;
    else
      cout << ' ';
  }
}
bool EQ(double a, double b) { return abs(a - b) < EPS; }
void fastStream() {
  cin.tie(0);
  std::ios_base::sync_with_stdio(0);
}
vector<string> split(string str, char del) {
  vector<string> res;
  for (int i = 0, s = 0; i < SIZE(str); i++) {
    if (str[i] == del) {
      if (i - s != 0) res.push_back(str.substr(s, i - s));
      s = i + 1;
    } else if (i == SIZE(str) - 1) {
      res.push_back(str.substr(s));
    }
  }
  return res;
}
int N;
int AS[1000100];
int main() {
  fastStream();
  cin >> N;
  for (int i = 0; i < N; i++) cin >> AS[i];
  int cnts[5] = {};
  int sum = 0;
  for (int i = 0; i < N; i++) {
    cnts[AS[i]]++;
    sum += AS[i];
  }
  if (sum == 1 || sum == 2 || sum == 5)
    cout << -1 << endl;
  else {
    if (cnts[2] == cnts[1])
      cout << cnts[1] << endl;
    else if (cnts[2] < cnts[1]) {
      int res = 0;
      res += min(cnts[1], cnts[2]);
      cnts[1] -= cnts[2];
      cnts[3] += cnts[2];
      cnts[2] = 0;
      res += cnts[1] / 3 * 2;
      cnts[3] += cnts[1] / 3;
      cnts[1] = cnts[1] % 3;
      if (cnts[1] % 3 == 1) {
        if (cnts[3] > 0)
          res++;
        else
          res += 2;
      } else if (cnts[1] % 3 == 2) {
        if (cnts[4] > 0)
          res += 2;
        else
          res += 2;
      }
      cout << res << endl;
    } else {
      int res = 0;
      res += min(cnts[1], cnts[2]);
      cnts[2] -= cnts[1];
      cnts[3] += cnts[1];
      cnts[1] = 0;
      res += cnts[2] / 3 * 2;
      if (cnts[2] % 3 == 1) {
        if (cnts[4] > 0)
          res += 1;
        else
          res += 2;
      } else if (cnts[2] % 3 == 2) {
        res += 2;
      }
      cout << res << endl;
    }
  }
  return 0;
}
