program cch(input,output);
const
 maxm=10000000000;
var
 a:array[1..8,1..8] of integer;
 ans:array[1..20] of longint;
 num:array[1..8,1..8,1..8,1..8] of longint;
 i,j,k,p,r,w,n:integer;
 min,x0:real;

procedure search(k,x1,x2,y1,y2:integer);
var
 i,j:integer;
 tot:real;
begin
 if k=n then
  begin
   ans[k]:=num[x1,x2,y1,y2];
   tot:=0;
   for j:=1 to n do
    tot:=tot+sqr(ans[j]-x0);
   if sqrt(tot/n)<min then min:=sqrt(tot/n);
  end;
 for i:=x1 to x2-1 do
  begin
   ans[k]:=num[x1,i,y1,y2];
   search(k+1,i+1,x2,y1,y2);
  end;
 for i:=y1 to y2-1 do
  begin
   ans[k]:=num[x1,x2,y1,i];
   search(k+1,x1,x2,i+1,y2);
  end;
end;

begin
 assign(input,'division.in');
 assign(output,'division.out');
 reset(input);
 rewrite(output);
 readln(n);
 for i:=1 to 8 do
  for j:=1 to 8 do read(a[i,j]);
 for i:=1 to 8 do
  for j:=i to 8 do
   for k:=1 to 8 do
    for p:=k to 8 do
     begin
      num[i,j,k,p]:=0;
      for r:=i to j do
       for w:=k to p do
        inc(num[i,j,k,p],a[r,w]);
     end;
 x0:=num[1,8,1,8]/n; min:=maxm;
 search(1,1,8,1,8);
 write(min:0:3);
 close(input);
 close(output);
end.