#include<iostream>

int main()
{
	long long Long,count=1,Max=0;
	
	scanf("%lld",&Long);
	
	long long bag[Long];
	
	for(long long i=0;i<Long;i++)
		scanf("%lld",&bag[i]);
	
	for(long long i=1;i<Long;i++)
	{
		if(count>=Max)
			Max=count;
		if(bag[i]==bag[i-1]+1)
			count++;
		else
			count=1;
	}
	
	printf("%lld",Max);
	
	return 0;
}