program p1;
var f,g:text;
    n,m,s:longint;
begin
assign(f,'fact.in');reset(F);
assign(g,'fact.out');rewrite(g);
readln(f,n);
m:=5;
s:=0;
while m<=n do begin
                s:=s+(n div m);
                m:=5*m;
                end;
write(g,s);
close(f);
close(G);

end.