Program factorial;
Type restrictie=1..100000000;
Var fi,fo:text;
    p:restrictie;
    n:int64;
    i,k:integer;
Begin
assign(fi,'factorial.in');
assign(fo,'factorial.out');
reset(fi);
readln(fi,p);
close(fi);
rewrite(fo);
k:=1;
n:=1;
for i:=1 to p do k:=10*k;
repeat            begin
                  n:=n*i;
                  inc(i);
                  end;
until n mod k=0;
Writeln(fo,i-1);
close(fo);
end.