#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (n); i++)
using namespace std;
typedef long long ll;

int main(){
    cin.tie(0);
    ios::sync_with_stdio(0);

    int N; cin >> N;
    vector<int> A(N);
    rep(i,N) cin >> A[i];
    sort(A.begin(), A.end());

    int c = 0, now = 0;
    rep(i,N-1){
        if(A[i+1] = A[i] + 1){
            now++;
            c = max(c, now);
        }else{
            now = 0;
        }
    }

    cout << c << endl;
}