What does this C++ snippet output?

1#include <iostream>
2#include <unordered_map>
3int main() {
4    std::unordered_map<std::string,int> m;
5    m["x"] = 5;
6    m["x"] += 3;
7    std::cout << m["x"] << " " << m.size() << "\n";
8}