program factorial;
var pr,p,nr,n:longint;
    f,g:text;
begin
assign(f,'factorial.in');reset(f);
assign(g,'factorial.out');rewrite(g);
readln(f,p);
pr:=1;
nr:=0;
n:=1;
while nr<>p do begin
               pr:=pr*n;
               if n mod 10=0 then begin
                                  nr:=nr+1;
                                  pr:=n div 10 mod 10;
                                  end
               else pr:=pr mod 10;
               n:=n+1;
               end;
writeln(g,(n-1) div 2);
close(f);close(g);
end.