program factory(input,output);   // gao jing du(X) int64 ke yi jie jue wen ti ;
 var
  i,j,k:longint;
  n:longint;
  s:longint;
  min,now:int64;
  temp:int64;
  c:array [1..5000] of int64;
  y:array [1..10000] of int64;
  ans:int64;


begin
 assign(input,'factory.in');
 assign(output,'factory.out');
 reset(input);
 rewrite(output);
  readln(n,s);
 for i:=1 to n do
  readln(c[i],y[i]);
 ans:=c[1]*y[1];
  for i:=2 to n do begin
   min:=2147000000;
    for j:=1 to i-1 do
     begin
      now:=(c[j]+(i-j)*s)*y[i];
      if (now<min) then min:=now;
      end;
     if c[i]*y[i]<min then min:=c[i]*y[i];
      ans:=ans+min;
    end;
 writeln(ans);

 close(input);
 close(output);
end.