#include <bits/stdc++.h>
#define int long long
using namespace std;
int n, x, v, sum, ans, res1, res2, res3, a[100005];
signed main()
{
  std::ios::sync_with_stdio(false), cin.tie(0), cout.tie(0);
  cin >> n;
  for (int i = 1; i <= n; i++)
  {
    cin >> a[i], sum += a[i];
    if (a[i] > a[res1])
      res3 = res2, res2 = res1, res1 = i;
    else if (a[i] > a[res2])
      res3 = res2, res2 = i;
    else if (a[i] > a[res3])
      res3 = i;
  }
  swap(a[1], a[res1]), swap(a[2], a[res2]), swap(a[3], a[res3]);
  if (a[1] * 2 > sum - a[1])
  	cout << -1, exit(0);
  for (int i = 1; i <= n; i++)
    if (a[i] & 1)
      x++, a[i]--, sum--;
  if ((sum - a[1]) / 2 >= a[1])
  {
  	v = sum / 6;
	x += sum - v * 6;
  }
  else if (sum - a[1] - a[2] >= a[2])
  {
  	a[2] = a[3] = (sum - a[1]) / 2;
  	if (a[2] & 1)
      x += 2, a[2]--, a[3]--;
    x += a[1] - a[2];
    v = a[2] / 2;
  }
  else
  {
  	a[3] = sum - a[1] - a[2];
  	v = a[3] / 2;
  	x += a[1] + a[2] - 2 * a[3];
  }
  cout << min(v, v - (int)ceil((3 * v - x) / 9.0));
  exit(0);
}

代码如上，最优解