Program Factorial;

var p,n:longint;
    z:boolean;
    f,k:longint;
    i,o:text;

begin
    assign(i,'fact.in');
    assign(o,'fact.out');
    reset(i);
    rewrite(o);
    Read(i,p);

     n:=1;f:=1;
     z:=true;
    While z do begin
                                       f:=f*n;
                                       inc(n);
                                       k:=f div 5;
                                       if (k>100000000) then z:=false else;
                                         end;


    if  (k=p) then Write(o,n)
              else Write(o,-1);

    close(i);
    close(o);
end.