#include <bits/stdc++.h>
using namespace std;
int n, x, res;
map<int, int>mp;

int main() {
	cin >> n;
	for (int i = 1; i <= n; i++) {
		cin >> x;
		mp[x]++;
	}
	int last = -1;
	for (auto t : mp) {
		if (t.second == 0)
			continue;
		res += t.first - last - 1;
		mp[t.first]--;
		if (mp[t.first] % 2 == 1)
			mp[t.first]++, res++;
		mp[t.first + 1] += mp[t.first] / 2;
		mp[t.first] = 0;
		last = t.first;
	}
	cout << res;
	return 0;
}