#include<stdio.h>
int main()
{
	int n, count = 0, ans = 0, temp = 0;
	int a[10000];
	scanf("%d", &n);
    	if (n == 1)
	{
		printf("1");
		return 0;
	}
	for (int i = 0;i < n;i++)
	{
		scanf("%d", &a[i]);
	}
	for (int i = 0;i < n - 1;i++)
	{
		if (a[i + 1] - a[i] == 1)
			count++;
		else
		{
			if (count > temp)
			{
				temp = count;
				ans = ++count;
			}
			count = 0;
		}
	}
	printf("%d", ans);
	return 0;
}