Iterating through a map containing set, map

Here is the code that I wanted to optimize,

Var = std::map<std::string, std::set<std::string>>;

Var has a key and set of values, now I wanted to find how many times a value is used in the overall map and I wrote the following,

std::map<std::string, int> Rate;
    for (const auto& key : Var)
    {
        for (const auto& par : key.second)
        {
            Rate[key.first] = key.second.size();
        }
    }

Now, I am wondering if I can reduce the above loops into a single loop or any other optimization possible.