#include<iostream>
#include <algorithm>
using namespace std;
int a[10005];
int main()
{
	int n;
	int maxn = 0;
	cin >> n;
	for(int i = 1;i <= n;i++)
		cin >> a[i];
	for(int i = 1;i <= n - 1;i++)
	{
		int s = 0;
		for(int j = i ;j <= n ;j++)
		{
			if(a[j] == a[j-1] + 1)
				s++;
			else
				break;
		}
		maxn = max(maxn,s);
	}
	cout << maxn + 1;
	return 0;
}