vector<int64_t> subsets(n + 1); 
  subsets[0] = 1; // empty set 

  // runs in O(2 ^ n * n) 
  for (int mask = 1; mask < (1 << n); mask += 1) {
    int subsetSize = __builtin_popcount(mask); 
    int64_t count = 1; 
    for (int j = 0; j < n; j++) {
      if (mask & (1 << j)) 
        count *= v[j]; // v[j] == size of j'th disjoint set
    }
    subsets[subsetSize] += count; 
  } 

  int64_t ans = 0; 
  for (int sz = 0; sz <= n; sz++) {
    ans += subsets[sz]; 
  }

  cout << ans << "\n";