# include<iostream>
#include<algorithm>
# include <cstring>
#include<math.h>
using namespace std;

int main()
{	
	int n,t=1,m=1;
	cin >> n;
	int a[1003];
	for(int i=0;i<n;i++){
		cin >> a[i];
	}

	for(int i=0;i<n-1;i++){
		if(a[i]+1==a[i+1]){
			t++;
		}
		else {
			if(m<t) m = t;
			t=1;	
		}
	}

	cout << m;
	
    return 0;
 }