import java.math.BigInteger;
import java.util.Arrays;
import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		new Main().run();
	}

	void run() {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		if (!(1 <= n && n <= 1000))
			throw new AssertionError();
		double sum = 0;
		for (int i = 0; i < n; ++i) {
			double x = sc.nextDouble();
			double a = sc.nextDouble();
			if (!(Math.abs(a) <= 1 && 0 <= x && x <= 2 * Math.PI))
				throw new AssertionError();
			sum += a;
		}
		System.out.println(sum * Math.PI);
	}

	static void tr(Object... objects) {
		System.out.println(Arrays.deepToString(objects));
	}
}