#include<stdio.h>
#include<math.h>
int main()
{
	int n, i, j, k, max = 0;
	int a[10005], cnt = 0;
	scanf_s("%d", &n);
	for (i = 0; i < n; i++)
		scanf_s("%d", &a[i]);
	for (j = 0; j < n; j++)
	{
		for (k = 0; k < n; k++)
		{
			if (a[k] - a[j] == k - j)
			{
				if (k - j >= max)
					max = k - j;
			}
		}
	}
	printf("%d\n", max + 1);
	return 0;
}