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();
		double sum = 0;
		for (int i = 0; i < n; ++i) {
			double x = sc.nextDouble();
			double a = sc.nextDouble();
			for (int j = -10000; j <= 10000; ++j) {
				sum += a * Math.sin(x + j) / (x + j);
			}
		}
		System.out.println(sum);
	}

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