var r1,p,i,p5,nr0,r,rest:longint;
    f1,f2:text;
begin
  assign(f1,'fact.in');
  reset(f1);
  assign(f2,'fact.out');
  rewrite(f2);
  read(f1,p);
  i:=0;
  p5:=1;
  nr0:=0;

  repeat
    nr0:=nr0+p5;
    i:=i+1;
    p5:=p5*5;
  until nr0+p5>p;
  r:=p div nr0*p5;

  rest:=p mod nr0;

  i:=0;
  p5:=1;
  nr0:=0;
  repeat
    nr0:=nr0+p5;
    i:=i+1;
    p5:=p5*5;
  until nr0+p5>rest;
  r:=r+rest div nr0*p5;

  r1:=r;
  p:=p+1;
   i:=0;
  p5:=1;
  nr0:=0;

  repeat
    nr0:=nr0+p5;
    i:=i+1;
    p5:=p5*5;
  until nr0+p5>p;
  r:=p div nr0*p5;

  rest:=p mod nr0;

  i:=0;
  p5:=1;
  nr0:=0;
  repeat
    nr0:=nr0+p5;
    i:=i+1;
    p5:=p5*5;
  until nr0+p5>rest;
  r:=r+rest div nr0*p5;
   if r=r1 then writeln(f2,-1)
           else writeln(f2,r1);
  close(f1);
  close(f2);
end.