program magica;
var
  n,p,q,o:longint;
  r,s,t:longint;
  i,j,k,l:longint;
  a:array[1..1000,1..3]of longint;
  money:longint;
procedure sort(const x,y:longint);
var
  i,j,k,l:longint;
begin
  i:=x;
  j:=y;
  k:=a[(x+y)div 2,3];
  repeat
    while a[i,3]<k do inc(i);
    while a[j,3]>k do dec(j);
    if i<=j then
    begin
      l:=a[i,1];
      a[i,1]:=a[j,1];
      a[j,1]:=l;
      l:=a[i,2];
      a[i,2]:=a[j,2];
      a[j,2]:=l;
      l:=a[i,3];
      a[i,3]:=a[j,3];
      a[j,3]:=l;
      inc(i);
      dec(j);
    end;
  until i>j;
  if x<j then sort(x,j);
  if i<y then sort(i,y);
end;
begin
  assign(input,'magica.in');
  reset(input);
  assign(output,'magica.out');
  rewrite(output);
  money:=0;
  readln(n,p);
  r:=0;
  for i:=1 to n do
  begin
    read(j);
    if not eoln then
    begin
      readln(k);
      if (k-j>p) then
      begin
        inc(r);
        a[r,1]:=j;
        a[r,2]:=k;
        a[r,3]:=k-j;
      end
      else
        money:=money+j;
    end
    else
    begin
      money:=money+j;
      readln;
    end;
  end;
  i:=1;
  if money<p then
  begin
    sort(1,r);
    while money <p do
    begin
      money:=money+a[i,1];
      inc(i);
    end;
  end;
  for j:=i to r do
  begin
    money:=money+a[j,2]-p;
  end;
  writeln(money);
  close(input);
  close(output);
end.