#nullable enable

#region
var (_input, _iter) = (Array.Empty<string>(), 0);
T I<T>() where T : IParsable<T>
{
    while (_iter >= _input.Length) (_input, _iter) = (Console.ReadLine()!.Split(' '), 0);
    return T.Parse(_input[_iter++], null);
}
#endregion

var n = I<int>();
if (n == 3)
{
    var ans =
"""
18
1 3
1 5
1 6
1 8
2 4
2 6
2 7
2 9
3 5
3 7
3 8
4 6
4 8
4 9
5 7
5 9
6 8
7 9
""";
    Console.WriteLine(ans);
}
else Console.WriteLine(-1);