#include <bits/stdc++.h>
#include <random>
using namespace std; typedef unsigned long long _ulong; typedef long long int lint; typedef pair<lint, lint> plint; typedef pair<double long, double long> pld;
#define ALL(x) (x).begin(), (x).end()
#define SZ(x) ((lint)(x).size())
#define FOR(i, begin, end) for(lint i=(begin),i##_end_=(end);i<i##_end_;i++)
#define IFOR(i, begin, end) for(lint i=(end)-1,i##_begin_=(begin);i>=i##_begin_;i--)
#define REP(i, n) FOR(i,0,n)
#define IREP(i, n) IFOR(i,0,n)
#define endk '\n'
template<class T>bool chmax(T& a, const T& b) { if (a < b) { a = b; return 1; } return 0; }
template<class T>bool chmin(T& a, const T& b) { if (b < a) { a = b; return 1; } return 0; }
lint gcd(lint a, lint b) { if (b == 0) return a; else return gcd(b, a % b); }
lint ceil(lint a, lint b) { return (a + b - 1) / b; }
lint digit(lint a) { return (lint)log10(a); }
const lint MOD = 1e9 + 7, INF = 1e18;
lint dx[8] = { 1, 0, -1, 0, 1, -1, 1, -1 }, dy[8] = { 0, 1, 0, -1, -1, -1, 1, 1 };
void YN(bool flag) { cout << (flag ? "YES" : "NO") << endk; }
typedef pair<lint, string> Pa;
typedef pair<lint, plint> tlint;
struct edge {
	lint v;
	lint a, b;
};

lint N, A;
int main() {
	cin >> N;
	set<lint> st;
	REP(i, N) {
		cin >> A;
		st.insert(A);
	}
	lint maxv = 0, prv = -1, cnt = 0;
	for (lint a : st) {
		if (prv + 1 == a) {
			cnt++;
			chmax(maxv, cnt);
		}
		else {
			cnt = 1;
		}
		prv = a;
	}
	cout << maxv << endk;
}