#include<iostream>
#include<vector>
#include<functional>
#include<algorithm>
using namespace std;
int p[10000];
int main()
{
	vector<int>v;
	int l, count = 1;
	cin >> l;
	for (int i = 0; i < l; i++)
		cin >> p[i];
	for (int i = 0; i < l; i++)
	{
		if (p[i + 1] - p[i] == 1)
		{
			count++;
			if (p[i + 2]-p[i + 1] != 1)
			{
				v.push_back(count);
				i++;
				count = 1;
			}
		}
	}
	sort(v.begin(), v.end(), greater<int>());
	cout << v[0];
	return 0;
}