Program factorial;
VAR
  Be,Ki: Text;
  p,i: Longint;

Function fact(x: Longint): Int64;
Begin
if x=0 then fact:=1
else
fact:=x*fact(x-1);
End;

Function hatvany(a,b:Longint): Longint;
var i:Longint;
Begin
for i:=1 to b do a:=a*a;
hatvany:=a;
End;

BEGIN

Assign(Be,'factorial.in'); Reset(Be);
Assign(Ki,'factorial.out'); Rewrite(Ki);

Read(Be,p);

i:=4;
Repeat inc(i)
Until
fact(i) mod (hatvany(10,p)) = 0;

Write(Ki,i div 2);

Close(Be); Close(Ki);

END.