typedef pair<int, int> PII;

struct pair_hash
{
    template<class T1, class T2>
    size_t operator() (const pair<T1, T2>& p) const
    {
        auto h1 = hash<T1> {}(p.first);
        auto h2 = hash<T2> {}(p.second);
        return h1 ^ h2;
    }
};

unordered_map<PII, int, pair_hash> hs;