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#include <cstdio>

const int M = 203000;

int t[M];

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