/**
 * for练习
 */
public class Main {

	public static void main(String[] args) {
		double pi = 0;
		for (int i = 1; i < 100000; i += 4) {
			// TODO:
			pi = pi - 1.0/(i+2) + 1.0/(i+4);
		}
		System.out.println(4*(pi+1));
	}

}