C++ function pointer syntax. Why does (*) work but * not?

I am writing a program for an assignment for a post fix calculator. There are bonus points if I can do it without using an if statement (no switch or while or any comparisons). I was reading about function pointers and came up with the following code:

int main(){
    unordered_map<char,int*(int,int)> m;
    m.insert({'+',[](int const a, int const b){return a+b;}});
}

This doesn't work.

However this does:

int main(){
    unordered_map<char,int(*)(int,int)> m;
    m.insert({'+',[](int const a, int const b){return a+b;}});
}

Why are (*) and * different?