#include <array>
#include <cstdint>
#include <iostream>
#include <stdexcept>

#if 1 // Not in C++11 // make_index_sequence

template <std::size_t...> struct index_sequence {};

template <std::size_t N, std::size_t... Is>
struct make_index_sequence : make_index_sequence<N - 1, N - 1, Is...> {};

template <std::size_t... Is>
struct make_index_sequence<0u, Is...> : index_sequence<Is...> {};

#endif // make_index_sequence

namespace detail
{

template <std::size_t N>
struct Fibo :
    std::integral_constant<size_t, Fibo<N - 1>::value + Fibo<N - 2>::value>
{
    static_assert(Fibo<N - 1>::value + Fibo<N - 2>::value >= Fibo<N - 1>::value,
                  "overflow");
};

template <> struct Fibo<0u> : std::integral_constant<size_t, 0u> {};
template <> struct Fibo<1u> : std::integral_constant<size_t, 1u> {};

template <std::size_t ... Is>
constexpr std::size_t fibo(std::size_t n, index_sequence<Is...>)
{
    return const_cast<const std::array<std::size_t, sizeof...(Is)>&&>(
        std::array<std::size_t, sizeof...(Is)>{{Fibo<Is>::value...}})[n];
}

template <std::size_t N>
constexpr std::size_t fibo(std::size_t n)
{
    return n < N ? fibo(n, make_index_sequence<N>()) : throw std::runtime_error("out of bound");
}
} // namespace detail

constexpr std::size_t fibo(std::size_t n)
{
    // 48u is the highest
    return detail::fibo<48u>(n);
}

int main()
{
    std::cout << fibo(42) << std::endl;
    std::size_t input;
    std::cin >> input;
    std::cout << fibo(input) << std::endl;
    
    return 0;
}