#include <iostream>
#include <cmath>

int main() {
    double value = 2.0;
    double result = acos(value);
    if (std::isnan(result)) {
        std::cout << "Input is out of range [-1, 1]" << std::endl;
    } else {
        std::cout << "acos(" << value << ") = " << result << " radians" << std::endl;
    }
    return 0;
}