uses math;
var st,sf:array[0..5000]of longint;
    f:array[1..5001]of longint;
    i,j,k,l,m,n,s,t,ff:longint;
begin
  assign(input,'batch.in');
  assign(output,'batch.out');
  reset(input);rewrite(output);
  readln(n);
  readln(s);
  for i:=1 to n do
    begin
      readln(t,ff);
      st[i]:=st[i-1]+t;
      sf[i]:=sf[i-1]+ff;
    end;
  for i:=n downto 1 do
    begin
      f[i]:=maxlongint;
      for j:=i to  n do
        begin
          f[i]:=min(f[i],f[j+1]+(st[j]-st[i-1]+s)*(sf[n]-sf[i-1]));
        end;
    end;
  writeln(f[1]);
  close(input);close(output);
end.