program ex(f1,f2);
 type
  zn=array [1..20000] of longint;
 var
  s,w:zn; i,j,k,m,n,l,p:longint;
  f1,f2:text; ans,now:int64;
 begin
  assign(f1,'factory.in'); reset(f1);
  assign(f2,'factory.out'); rewrite(f2);
  readln(f1,n,p);
  for i:=1 to n do readln(f1,w[i],s[i]);
  ans:=0;
  for i:=n downto 1 do
   begin
    now:=99999999999999999;
    for j:=i downto 1 do
      if (s[i]*w[j]+(i-j)*p*s[i])<now then now:=s[i]*w[j]+(i-j)*p*s[i];
    ans:=ans+now;
   end;
  writeln(f2,ans);
  close(f1);
  close(f2);
 end.