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#include <algorithm>
#include <cstdio>
#include <vector>

int main(void) {
  int n;

  // 201718 + 20 should be enough, but...
  std::vector<int> coins(300000);
  int range = 0;
  int power;

  // Read the input and remember the highest value
  std::scanf("%d", &n);
  while (n--) {
    std::scanf("%d", &power);
    ++coins[power];
    range = ::std::max(range, power);
  }

  // Get the highest achievable power
  int max_pow = 0;
  int rem = 0;
  for (; max_pow < range || (rem + coins[max_pow]) > 1; ++max_pow) {
    rem = (rem + coins[max_pow]) / 2;
  }

  std::printf("%d\n", max_pow);

  return 0;
}