What does this C++ snippet output?

1#include <iostream>
2 
3struct S {
4  int operator()(int a, int b) { return a - b; }
5};
6int apply(S f, int x, int y) { return f(x, y); }
7int main() {
8    std::cout << apply(S{}, 10, 3) << "\n";
9}