program ex;
type
 sz=array[1..10000]of longint;
 ss=array[0..100]of longint;
var
 f,c,y:sz;
 f1,f2:text;
 n,s:integer;
 t:ss;
procedure init;
 var
  i,j:integer;
 begin
  assign(f1,'factory.in');
  assign(f2,'factory.out');
  reset(f1);
  rewrite(f2);
  readln(f1,n,s);
  for i:=1 to n do
   readln(f1,c[i],y[i]);
  close(f1);
 end;
procedure main;
 var
  i,j,x,k:longint;
 begin
  x:=0;
  k:=1;
  f[1]:=c[1]*y[1];
  for i:=2 to n do
     if (c[k]+s*(i-k))<c[i] then f[i]:=(c[k]+s*(i-k))*y[i]
                            else begin
                                  f[i]:=c[i]*y[i];
                                  k:=i;
                                 end;
 end;
procedure add;
 var
  i,j,x:longint;
 begin
  for i:=1 to n do
   begin
    j:=1;
    t[1]:=t[1]+f[i];
    x:=f[i];
    while x>=10 do
     begin
      t[j+1]:=t[j+1]+t[j] div 10;
      t[j]:=t[j] mod 10;
      x:=x div 10;
      inc(j);
     end;
    if j>t[0] then t[0]:=j;
   end;
 end;
procedure print;
 var
  i:integer;
 begin
  add;
  for i:=t[0] downto 1 do
   write(f2,t[i]);
 end;
begin
 init;
 main;
 print;
 close(f2);
end.