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

vector<set<int> > a;
set<int> t;
double ans;
int n, m;

int main()
{
	a.push_back(t);
	scanf("%d%d", &n, &m);
	for (int i = 0, x; i < n; i++)
	{
		t.clear();
		for (int j = 0; j < m; j++)
		{
			scanf("%d", &x);
			t.insert(x);
		}
		a.push_back(t);
	}
	for (int i = 1; i <= n; i++)
	{
		a[i].erase(a[i].begin());
		a[i].erase(--a[i].end());
	}
	double p;
	for (int i = 1; i <= n; i++)
	{
		p = 0;
		for (set<int>::iterator it = a[i].begin(); it != a[i].end(); it++)
			p += *it;
		p = p * 1.0 / a[i].size();
		ans = max(ans, p);
	}
	printf("%.2lf", ans);
 	return 0;
}