#include <bits/stdc++.h>
using namespace std;

int main() {
	int n, max = 0, c;
	vector<int> a;
	cin >> n;
	for (int i = 1; i <= n; i++) {
		cin >> a[i];
	}
	for (int i = 1; i <= n; i++) {
		if (a[i] - a[i - 1] == 1) {
			c++;
		} else {
			if (c > max) {
				max = c;
				c = 0;
			}
		}
	}
	cout << max;
	return 0;
}