program bly(input,output);
var
  a,b:array[1..10000] of integer;
  f:array[1..10000] of int64;
  n,s:integer;
  ans,s0:int64;
  i,j:integer;
  f1,f2:text;

  function min(x,y:int64):int64;
  begin
    if x>y then min:=y
           else min:=x;
  end;
begin
  assign(f1,'factory.in');
  reset(f1);
  readln(f1,n,s);
  for i:=1 to n do readln(f1,a[i],b[i]);
  close(f1);

  for i:=1 to n do begin
    f[i]:=a[1]+s0;
    s0:=s0+s;
  end;
  for i:=2 to n do begin
    s0:=0;
    for j:=i to n do begin
      f[j]:=min(f[j],a[i]+s0);
      s0:=s0+s;
    end;
  end;
  ans:=0;
  for i:=1 to n do begin
    ans:=ans+f[i]*b[i];
  end;
  assign(f2,'factory.out');
  rewrite(f2);
  writeln(f2,ans);
  close(f2);

end.