import java.util.Scanner;
import java.util.Arrays;
public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
Scanner in=new Scanner(System.in);
int n=in.nextInt();
int a[]=new int [n];
int b[]=new int[n];
int sum=0;
int m=0;
for(int i=0;i<n;i++)
{
	int j=in.nextInt();
	a[i]=j;
}
Arrays.sort(a);
out:
for(int i=1;i<n;i++)
{
	if(a[i-1]==a[i])//如果两个相同的话
	{
		continue out;
	}
	if(a[i]==a[i-1]+1)
	{  
		sum++;
		b[m]=sum;
	}
	else
	{ 
		sum=0;
        m++;
	}
   
}
Arrays.sort(b);

System.out.print(b[b.length-1]);

	}

}