How to constrain my template to only accept lambda with specific input & output type?

Inspired by other question to calculate taylor series of a function(Original question), I wrote a template without any constraint to successfully calculate the sum. Here is current code (Template body removed, as @Elliott says it’s irrelevant to the point..):

#include <iostream>
#include <cmath>
#include <limits>

template<typename ftn>
long double Taylor_sum(ftn term) { /* Summation calculation goes here... */ return result; };

int main(){
    using namespace std;
    long double x;  cin >> x ;
    long double series_sum = Taylor_sum([x](unsigned long long int i) -> long double { return powl(-1.0L,i)*powl(x,2ULL*i+1ULL)/tgammal(2ULL*i+2ULL); });
    if (!isfinite(series_sum)) cout << "Series does not converge!" << endl;
    else {
        cout << "Series converged, its value is : " << series_sum << endl;
        cout << "Compared to sin                : " << sinl(x) << endl;
    }
}

Although the code works enough, to study & practice the concept myself, I am trying to constrain the template to accept only lambda with unsigned long long int as a input, and long double as output. Here is my current attempt (which does not compile):

template<typename T,integral ARG>
concept my_lambda = requires(T t, ARG u) {
    { return t(u); };
}

template<my_lambda ftn>
long double Taylor_sum(ftn term) { //The rest is same...

I googled various sources, but it seems to me that because the concept is relatively new feature in C++20, there seems less material available. Does anyone know how to constrain my template parameter properly?

>Solution :

I am trying to constrain the template to accept only lambda with
unsigned long long int as a input, and long double as output.

You can use compound requirements with return-type-requirement:

template<typename F>
concept my_lambda = requires(F f, unsigned long long int x) {
    { f(x) } -> std::same_as<long double>;
};

template<my_lambda ftn>
long double Taylor_sum(ftn term) { //The rest is same...

Demo