#include<stdio.h>
int main(void)
{
/Begin/
int n,x0 = 0,x1,i = 0,max = 0;
scanf("%d",&n);
do
{
scanf("%d",&x1);
n--;
if(x1 == x0 + 1)
i++;
else
{
if(i > max)
{
max = i;
i = 1;
}
}
x0 = x1;
}while(n != 0);
printf("%d",max)
/*********End**********/
return 0;
}