#include<stdio.h>
float dx(int m);

int main()
{
	int n,m;
	scanf("%d%d",&n,&m);
	int i;
	float num=0;
	for(i=1;i<=n;i++)
	{
		if((dx(m)/(m-2))>num)
		{
			num=dx(m)/(m-2);
			//printf("num=%f\n",num);
		}
	}
	printf("%.2f",num);
	return 0;
}
float dx(int m)
{
	int a[25];
	int i,max=-1,min=11;
	float count;
	for(i=0;i<m;i++)
	{
		scanf("%d",&a[i]);
		if(a[i]>max)
		{
			max=a[i];
			//printf("max=%d\n",max);
		}
		if(a[i]<min)
		{
			min=a[i];
			//printf("min=%d\n",min);
		}
	}
	for(i=0,count=0;i<m;i++)
	{
		count+=a[i];
		//printf("count=%f\n",count);
	}
	count-=(max+min);
	//printf("count=%f\n",count);
	return count;
}