import std;

int factorial(int n)

{

    return n.predSwitch!"a <= b"(

        -1, {throw new Exception("Can not calculate n! for n < 0");}(),

        0, 1, // 0! = 1

        n * factorial(n - 1) // n! = n * (n - 1)! for n >= 0

        );

}

void main() {

  writeln(factorial(3));

}