#include <cstdio>
#include <cmath>

int		main(void)
{
	int		n;
	int		i, j;
	int		v[100010];
	long long	maxi, result;

	maxi = v[2] + v[1];
	scanf("%d", &n);
	for (i = 0; i < n; i++)
		scanf("%d", &v[i]);
	for (i = 0; i < n - 1; i++)
	{
		for (j = i + 1; j < n; j++)
		{
			result = v[i] + v[j] - floor(log2(j - i));
			if (result > maxi)
				maxi = result;
		}
	}
	printf("%lld\n", maxi);
	return (0);
}