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#include <iostream>
using namespace std;
int main(){
	int n, t, m;
	int a[230000];
	m = -1;
	
	cin >> n;
	for(int i = 0; i < 230000; i++) a[i] = 0;
	for(int i = 0; i < n; i++){
		cin >> t;
		a[t]++;
	}	
	for(int i = 1; i < 230000; i++){
		a[i] += a[i - 1]/2;
		if(a[i] > 0) m = i;
	}
	
	cout << m << endl;
}