#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;
double jun(int*ans,int ted)
{
	double sum = 0;
	for (int i = 1; i <= ted; i++)
	{
		cin >> ans[i];
	}
	sort(ans[1], ans[ted]);
	for (int i = 2; i <= ted - 1; i++)
	{
		sum += ans[i];
	}
	return sum / (ted - 2);
}
int main()
{
	double maxn;
	int std, tea;
	cin >> std >> tea;
	int ans[21];
	for (int i = 1; i <= std; i++)
	{
		if (i == 1 || jun(ans, tea) > maxn) maxn = jun(ans, tea);
	}
	printf("%.2f", maxn);
	return 0;
}