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#include <cstdio>
#include <vector>
#include <algorithm>
#include <set>
#include <iostream>
#include <map>


using namespace std;

#define REP(i,n) for(int i=0; i<(int)n; i++)
#define ST first
#define ND second
#define MP make_pair
#define PB push_back

typedef vector<int> VI;
typedef pair<int,int> PI;
typedef vector<PI> VPII;
typedef set<int> SI;
typedef long long LL;

#ifdef DEBUG
const bool debug = true;
#else
const bool debug = false;
#endif
int n,m,k,l, test = 1;
const int inf = 1000 * 1000 * 1000 ;
const int MAKSN = 1000 * 1000 + 13; // UZUPElnic


int t[MAKSN];

void readIn()
{
	cin >> n;
	REP(i, n)
	{
		cin >> k;
		t[k]++;
		//Oczywko ze swietne
		while(t[k] >= 2) {
			t[k] -= 2;
			t[k+1]++;
			k++;
		}
	}
}


void solve()
{
	int ans = -1;
	REP(i, MAKSN) {
		if(t[i])
			ans = i;
	}

	cout << ans << "\n";
}

void zeruj()
{
	REP(i, MAKSN)
	t[i] = 0;
}

int main()
{
	ios_base::sync_with_stdio(0);

	// cin >> test;
    // REP(i, test)
    // {
    zeruj();
    readIn();
    solve();
    // }

    return 0;
}