program p1;
 const
  maxn=10000;
 var
     a,y:array[1..maxn] of longint;
     n,s,i,j,tm,min:longint;
     ans:int64;

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