#include <cassert>
#include <concepts>

template<int Length, int Time>
struct Quantity {
    double value;
    explicit constexpr Quantity(double v) : value(v) {}
};

template<int L, int T>
constexpr Quantity<L, T> operator+(Quantity<L, T> a, Quantity<L, T> b) {
    return Quantity<L, T>{a.value + b.value};
}

template<int L1, int T1, int L2, int T2>
constexpr Quantity<L1 + L2, T1 + T2> operator*(
    Quantity<L1, T1> a, Quantity<L2, T2> b) {
    return Quantity<L1 + L2, T1 + T2>{a.value * b.value};
}

template<class A, class B>
concept Addable = requires(A a, B b) { a + b; };

using Length = Quantity<1, 0>;
using Duration = Quantity<0, 1>;
using Speed = Quantity<1, -1>;
static_assert(Addable<Length, Length>);
static_assert(!Addable<Length, Duration>);
static_assert(std::same_as<decltype(Speed{3} * Duration{4}), Length>);

int main() {
    const Speed speed{3};
    const Duration elapsed{4};
    const Length traveled = speed * elapsed;
    const auto total = traveled + Length{5};
    assert(traveled.value == 12);
    assert(total.value == 17);
}