According to the C++23 standard, what is the output of this program?

#include <iostream>

int f(int &a, int &b) {
  a = 3;
  b = 4;
  return a + b;
}

int main() {
  int a = 1;
  int b = 2;
  int c = f(a, a);
  std::cout << a << b << c;
}