const max=100;{n的最大值}

maxn=3;{间距的最大值}

var a:array[-maxn+2..max]of longint;

i,j,n:integer;

begin

readln(n);

for i:=-maxn+2 to -1 do a[i]:=0;

a[0]:=2;a[1]:=2;

for i:=2 to n do begin

a[i]:=a[i-maxn];

for j:=1 to maxn-1 do

a[i]:=a[i]+a[i-j];

end;

writeln(a[n]);

end.