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var
  i,j,s,n,m,x,y:longint;
  a,b:array[1..1000] of longint;
procedure qsort(l,r:longint);
var i,j,mid,t:longint;
begin
  i:=l;j:=r;mid:=b[(l+r) shr 1];
  repeat
    while b[i]<mid do inc(i);
    while b[j]>mid do dec(j);
    if i<=j then begin
      t:=b[i];b[i]:=b[j];b[j]:=t;inc(i);dec(j);
    end;
  until i>j;
  if l<j then qsort(l,j);
  if i<r then qsort(i,r);
end;
begin
  readln(n,s);
  readln(x,y);
  for i:=1 to n do readln(a[i],b[i]);
  qsort(1,n);
  i:=1;
  while s>b[i] do begin
    if x+y>a[i] then begin inc(m);s:=s-b[i];end;
    inc(i);
  end;
  write(m);
end.