Fast Fibonacci

fib.cpp

#include <iostream>
#include <iomanip>
#include <cmath>

int main() {
    int input = 0;
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    if (std::cin >> input) {
        double sqrt5 = std::sqrt(5);
        double left = (1. + sqrt5) / 2.;
        double right = (1. - sqrt5) / 2.;
        std::cout << std::fixed << std::setprecision(1)
                  << (std::pow(left, input) - std::pow(right, input)) / sqrt5
                  << "\n";
    }
    return 0;
}