Determine the output of the program with a function reference

It is necessary to understand what values will be printed when executing the following code.

#include <iostream>

int counter = 2;

int& refFunc(int x){
    counter = counter + x;
    return counter;
}

int main(){
    int val = 2;
    std::cout << refFunc(val) << std::endl;
    refFunc(val) = 10;
    std::cout << refFunc(val) << std::endl;
    val = refFunc(val);
    std::cout << refFunc(val) << std::endl;
}