#include <bits/stdc++.h>
using namespace std;
int a[100000];
int main() {
	ios::sync_with_stdio(false);
	cin.tie();
	cout.tie();
	int n;
	cin >> n;
	for (int i = 0; i < n; i++) cin >> a[i];
	sort(a, a + n);
	int minn = INT_MIN, cnt = 1;
	for (int i = 1; i < n; i++) {
		if (a[i] - a[i - 1] == 1) cnt++;
		else minn = cnt, cnt = 0;
	}
	cout << minn << endl;
	return 0;
}

20分，真奇怪。大佬帮忙指点一下