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

typedef long long int           ll;
typedef unsigned long long int  ull;
typedef vector<int>             vi;
typedef vector<ll>              vll;
typedef vector<vi>              vvi;
typedef pair<int,int>           pii;
typedef vector< pii >           vpii;
typedef set<int>                sti;

#define pb                      push_back
#define pob                     pop_back
#define mp                      make_pair
#define ff                      first
#define ss                      second
#define loop1(i,n)              for(int i=1; i<=int(n); i++)
#define loop(i,y)               for(int i=0; i<int(y); i++)
#define rloop(i,y)              for(int i=int(y); i>0; i--)
#define rloop1(i,y)             for(int i=int(y)-1; i>=0; i--)
#define read()                  freopen("input.txt", "r", stdin)
#define write()                 freopen("output.txt", "w", stdout)
#define cspf(i)                 printf("Case %d: ", i)
#define all(v)                  v.begin(),v.end()
#define rall(v)                 v.rbegin(),v.rend()
/// Constants
#define eps                     1e-9
#define PI                      acos(-1.0)  // 3.1415926535897932


int main()
{
    vi vt;
    int n,i,cnt=0,f1,cn1=0,mx=0;
    scanf("%d",&n);
    int a[n+5];
    loop1(i,n){
        scanf("%d",&a[i]);
    }
    cnt=0;
    for(i=1;i<n;i++){
        if(a[i+1]-a[i]==1){
            cnt++;
            continue;
        }
        if(cnt>0){
            if(a[i]==i)
                vt.pb(cnt);
            else
                vt.pb(cnt-1);
            cnt=0;
        }
    }
    if(cnt>0){
        if(a[n]==1000 || a[n]==n)
            vt.pb(cnt);
        else
            vt.pb(cnt-1);
    }
    if(vt.size()==0){
        printf("0\n");
        return 0;
    }
    sort(rall(vt));
    printf("%d\n",vt[0]);
    return 0;
}