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#include<iostream>
using namespace std;
//**************************************************************************************************************************
int wartosc[300000];
//**************************************************************************************************************************
int main()
{
	int a,n,bufor,max = -1;
	int licz = 1;

	cin >> n;

	for (int i = 0; i < n; i++)
	{
		cin >> a;
		wartosc[a]++;
	}

	for (int i = 0; i < 300000; i++)
	{
		if (wartosc[i] >= 2)
		{
			bufor = wartosc[i] / 2;

			wartosc[i] -= bufor * 2;

			wartosc[i + 1] += bufor;
		}
		
	}

	for (int i = 0; i < 300000; i++)
	{
		if (wartosc[i] == 1)
		{
			max = i;
		}

	}

	cout << max << endl;
	
	return 0;
}
//**************************************************************************************************************************