#include <bits/stdc++.h>
using namespace std;
int main()
{
	int n,m,m2,x,maxn=-999999,minn=999999;
	double sum=0,maxx=-999999;
	cin>>n>>m;
	m2=m-2;
	for(int i=1;i<=n;i++)
	{
		for(int j=1;j<=m;j++)
		{
			cin>>x;
			if(x>maxn) maxn=x;
			if(x<minn) minn=x;
			sum+=x;
		}
		sum-=maxn;
		sum-=minn;
		sum/=m2;
		maxx=max(sum,maxx);
	}
	printf("%.2lf",maxx);
	return 0;	
}