Binding a const l-value reference to r-value is fine but not vice versa

So, I've been studying about std::move() and I came across l-value and r-value reference and a question popped in my mind -

Consider a code here -:

#include <iostream>
int fun(int arg){
    return arg;
}
int main() {
    // Write C++ code here
    int a=10;
    std::cout<<fun(a);
    return 0;
}

Now if I want l-value arg to accept r-value I have to declare it const, like in this case -:

#include <iostream>
int fun(const int& arg){
    return arg;
}
int main() {
    // Write C++ code here
    int a=10;
    std::cout<<fun(a);
    std::cout<<fun(10); //both will work fine
    return 0;
}

But if I do vice-versa i.e. using r-value const

#include <iostream>
int fun(const int&& arg){
    return arg;
}
int main() {
    // Write C++ code here
    int a=10;
    std::cout<<fun(a);
    std::cout<<fun(10); //throwing error
    return 0;
}

Its showing error like -:

cpp:9:20: error: cannot bind rvalue reference of type 'const int&&' to lvalue of type 'int' 9 | std::cout<<fun(a); | ^ /tmp/ufgkAH4KAC.cpp:3:21: note: initializing argument 1 of 'int fun(const int&&)' 3 | int fun(const int&& arg){ | ~~~~~~~~~~~~^~~

Can anyone explain me why?