var a:array[1..100] of 1..1000000;
    f,g:text;
    n,m,fr,i:1..1000000;
begin
assign(f,'fractii.in');
assign(g,'fractii.out');
reset(f);
read(f,n);
a[1]:=1;
a[2]:=2;
for i:=3 to n do begin
    a[i]:=a[i-1]+a[i-2]
end;
i:=1;
repeat
      fr:=fr+a[i];
      m:=m+1;
      i:=i+1;
until m=n;
rewrite(g);
write(g,fr);
close(f);
close(g);
end.