//na                            :cuixiaofei;
//da                            :08_04_24;
program rez;
  var
    n,max                       :longint;
    a                           :array[-1..9000,-1..9010] of boolean;
    f                           :array[-1..9010] of longint;
    f1,f2                       :text;
  procedure init;
    var
      i,a1,a2                   :longint;
  begin
    assign(f1,'rez.in');
    reset(f1);
    assign(f2,'rez.out');
    rewrite(f2);
    fillchar(a,sizeof(a),false);
    fillchar(f,sizeof(f),0);
    max:=0;
    readln(f1,n);
    for i:=1 to n do
      begin
        readln(f1,a1,a2);
        a[a1,a2]:=true;
        if a2>max then
          max:=a2;
      end;
  end;
  procedure main;
    var
      i,j,mmm                  :longint;
  begin
    for i:=0 to max do
      begin
        mmm:=0;
        for j:=0 to i-1 do
          begin
            if a[j,i]=true then
              begin
                if f[j]+i-j>mmm then
                  mmm:=f[j]+i-j;
              end
            else
              begin
                if f[j]>mmm then
                  mmm:=f[j];
              end;
          end;
        if mmm>f[i] then
          f[i]:=mmm;
        if f[i]<f[i-1] then
          f[i]:=f[i-1];
      end;
  end;
  procedure print;
  begin
    writeln(f2,f[max]);
    close(f1);
    close(f2);
  end;
  begin
    init;
    main;
    print;
  end.