# include <stdio.h>
 # include <stdlib.h>
 
 int a[105];
 int b[105];
 int c[105];
 int m, n, i, j, k, p;
 
 void sort(int * pA, int len);
 
 int main()
 {
 	scanf("%d%d", &n, &m);
 	p = m + 1;
 	while(p--)
 	{
 	    for (i=0; i<m; ++i)
		    scanf("%d", &a[i]);
					   			
		sort(a, m);	
		for (i=0; i<n-1; ++i)
 	    {
		    b[0] = a[0];
		    b[i+1] = a[i+1]+b[i];		
	    }
	    
	    c[p-1] = b[m-2]-b[0];		    				 			 	 		
	}
	sort(c, n);
	printf("%.2lf", 1.0*c[0]/(m-2));	  
 	
 	return 0;
 } 
 void sort(int * pA, int len)
 { 
      for (i=0; i<len-1; ++i)
       {
	       for (j=0; j<len-i-1; ++j)
		   {
		         if (pA[j]<pA[j+1])
				 {
				 	 k = pA[j];
				 	 pA[j] = pA[j+1];
				 	 pA[j+1] = k;
				 	 k = 0;
				 }		 
		   } 
	   } 
 }