def s_metric(G):
    """Return the s-metric of graph.

    The s-metric is defined as the sum of the products deg(u)*deg(v)
    for every edge (u,v) in G.

    Parameters
    ----------
    G : graph
        The graph used to compute the s-metric.

    Returns
    -------       
    s : float
        The s-metric of the graph.

    References
    ----------
    .. [1] Lun Li, David Alderson, John C. Doyle, and Walter Willinger,
           Towards a Theory of Scale-Free Graphs:
           Definition, Properties, and  Implications (Extended Version), 2005.
           http://arxiv.org/abs/cond-mat/0501169
    """
    return sum([G.degree(u)*G.degree(v) for (u,v) in G.edges_iter()])