program t2014_2
var n,k:integer;
function s(n,k:integer):longint;
begin
if (n<k)or(k=0) then
s:=0
else if (k=1)or(k=n) then
s:=1
else
s:=s(n-1,k-1)+k*s(n-1,k)
end;
begin
readln(n,k);
writeln(s(n,k));
end.
输入：
5 3

输出：15