#include<bits/stdc++.h>
using namespace std;

double a[110];
int n,m;
double x,sum,ans,ave;
double mins,maxs;

double work(double m)
{
	mins=10,maxs=0,sum=0;
	for(int i=1;i<=m;i++)
	{
		cin>>x;
		sum+=x;
		 if(x<mins)		mins=x;
		 if(x>maxs)		maxs=x;
	}
	sum=sum-maxs-mins;
	return sum;
}

int main(){

	cin>>n>>m;
	
	for(int i=1;i<=n;i++)
	{
		a[i]=work(m)/(m*1.0-2.0);
	 } 
	for(int i=1;i<=n;i++)
	{
		ans=a[1];
		if(a[i]>ans)	ans=a[i];
	}
	
	printf("%.2f",ans);
	
	return 0;
}