program xmz;
var
 t:array[1..5000]of longint;
 f:array[1..5000]of longint;
 s:array[0..5000]of longint;
 ts:array[0..5000]of longint;
 x:array[1..5000,1..5000]of longint;
 a,b,c,n,ss:longint;
 f1,f2:text;
begin
 assign(f1,'batch.in');assign(f2,'batch.out');
 reset(f1);rewrite(f2);
 read(f1,n,ss);
 for a:=1 to n do
  read(f1,t[a],f[a]);
 s[n]:=f[n];ts[n]:=t[n];
 for a:=n downto 2 do
  begin
  s[a-1]:=s[a]+f[a-1];
  ts[a-1]:=ts[a]+t[a-1];
  end;
 for a:=1 to n do
  for b:=1 to n-a+1 do
   begin
   x[a,b]:=(ts[b]-ts[a+b]+ss)*(s[b]);
   for c:=1 to a-1 do
   if x[a,b]>x[c,b]+x[a-c,b+c] then
    x[a,b]:=x[c,b]+x[a-c,b+c];
   end;
  write(f2,x[n,1]);
 close(f1);close(f2);

end.