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#include <bits/stdc++.h>
using namespace std;
const int mxn = 5e5 + 10;

int n;
int t[mxn];
int wyn = -1;

int main()
{
	scanf("%d", &n);
	for(int i = 1; i <= n; i++)
	{
		int a;
		scanf("%d", &a);
		t[a]++;
	}
	int curr = 0;
	for(int i = 0; i < mxn; i++)
	{
		curr += t[i];
		if(curr % 2 == 1)
			wyn = i;
		
		curr /= 2;
	}
	printf("%d\n", wyn);
}