public class Main {

	public static void main(String[] args) {
		double pi = 0;
		int n = 1;
		for (int i = 1; i < 10000; i += 2) {
			// TODO:
			pi = pi + 1.0/i * n;
			n = (-1)*n;
		}
		System.out.println(pi*4);
	}

}