#include <bits/stdc++.h>
using namespace std;
int a[1000005];
int main() {
	int n, l = 1, Max = 0,j=1;
	cin >> n;
	for (int i = 1; i <= n; i++) {
		cin >> a[i];
	}
	for (int i = j; i <= n; i++) {
		for ( j = i + 1; j <= n; j++) {
			if (a[j] - a[i] != 1) {
				if (l > Max) {
					Max = l;
					l = 1;
				}
				break;
			}else{
				l++;
			}
		}
	}
	cout << Max;
	return 0;
}