program factorila;
var p,i,m,q,z:longint;
    f:text;
function putere(q:longint):longword;
var p,i:longint;
begin
p:=1;
for i:=1 to q do p:=p*5;
putere:=p;
end;
function zerou(i:longint):longword;
var s:longword;
begin
s:=0;
while i mod 5=0 do begin
                   inc(s);
                   i:=i div 5
                   end;
zerou:=s;
end;

begin
assign(f,'fact.in'); reset(f); read(f,p); close(f);
assign(f,'fact.out'); rewrite(f);
z:=0;
q:=1;
i:=0;
while z<p do
          begin
              i:=i+1;
              m:=putere(q);
              if i=m then begin z:=z+q; inc(q) end
                     else z:=z+zerou(i)
          end;
write(f,i);
close(f);
end.