var             s: qword;
    n, m, k, p, i: longint;

function c(n, k: longint): qword;
 var s: qword;
     i: longint;
 begin
  s := 1;
  for i := n - k + 1 to n do
   s := s * i;
  for i := 1 to k do
   s := s div i;
  c := s
 end;

begin
 read(n, m, k, p);
 s := 0;
 for i := p downto 0 do
  s := s + c(m, i) * c(n, k - i);
 write(s);
end.