#include <stdio.h>

int main()
{
	int n,count=1,max=1,b=1;
	int i,a[10000];
	scanf("%d",&n);
	
	for(i=0;i<n;i++)
	{
		scanf("%d",&a[i]);
		if(i>0&&a[i]==(a[i-1]+1))
		{
			count++;
			b=(max>count)?max:count;
		}
		else if(i>0&&a[i]!=(a[i-1]+1))
		{
			max=count;
			count=1;
		}	
	}
	printf("%d",b);
	return 0;
 }