var
 a,d:array[0..11000]of longint;
 i,j,k,l,m,n,t:longint;
 ans:int64;
 
procedure down(i:longint);
 var
  j,aa,dd:longint;
 begin
  aa:=a[i];
  dd:=d[i];
  while i*2<=n do
   begin
    j:=i*2;
    if (j<n)and(a[j+1]>a[j]) then inc(j);
    if a[j]>aa then
     begin
      a[i]:=a[j];
      d[i]:=d[j];
      i:=j;
     end else break;
   end;
  a[i]:=aa;
  d[i]:=dd;
 end;

begin
 assign(input,'fisha.in');reset(input);
 assign(output,'fisha.out');rewrite(output);
 read(n,m,t,k);
 for i:=1 to n do read(a[i],d[i]);
 for i:=n div 2 downto 1 do down(i);
 
 m:=m-t*(n-1);
 if m mod k=0 then m:=m div k-1 else m:=m div k;
 
 while m>0 do
  begin
   ans:=ans+a[1];
   a[1]:=a[1]-d[1];
   if a[1]<0 then a[1]:=0;
   dec(m);
   down(1);
  end;
 writeln(ans);
 close(input);close(output);
end.