#include<iostream>
using namespace std;
int main() {
	int n, i, k, num;
	int b[100] = { 0 }, j = 0;
	long long a[100000];
	cin >> n;
	for (i = 0; i < n; i++)
		cin >> a[i];
	for (i = 0; i < n - 1; i++) {
		num = 1;
		k = i;
		if (a[k + 1] == a[k] + 1) {
			while ((a[k + 1] == a[k] + 1) && k < n - 1) {
				num++;
				k++;
			}
			b[j++] = num;
		}
	}
	int max = 0;
	for (i = 0; i < j; i++) {
		if (b[i] > max)
			max = b[i];
	}
	cout << max;
	return 0;
}