#include<stdio.h>

int card[10000];
int used[10000] = {0};
int n, k;
int max = 0;

void search(int num, int cnt)
{
  int i;
  int z_flag = 0;

  for(i = 0; i < k; i++)
    {
      if(!used[i] && (card[num] + 1 == card[i] || card[i] == 0))
	{
	  used[i] = 1;
	  if(card[i] == 0)
	    {
	      card[i] = card[num] + 1;
	      z_flag = 1;
	    }
	  search(i, cnt + 1);
	  if(z_flag)
	    {
	      card[i] = 0;
	      z_flag = 0;
	    }
	  used[i] = 0;
	}
      if(cnt > max) max = cnt;
    }

  return;
}
       
int main(void)
{
  int i;

  scanf("%d%d", &n, &k);

  for(i = 0; i < k; i++)
    {
      scanf("%d", &card[i]);
    }

  for(i = 0; i < k; i++)
    {
      used[i] = 1;
      search(i, 1);
      used[i] = 0;
    }
  
  printf("%d\n", max);

  return 0;
}