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

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

	double sinc(double x) {
		if (x < 1e-8) {
			return 1;
		} else {
			return Math.sin(x) / x;
		}
	}

	double solve(double x, double a) {
		double ret = Math.cos(x + 0.5) / (x + 1) + Math.cos(-x + 0.5) / (-x + 1);
		for (int i = 1; i < 10000; ++i) {
			ret -= Math.cos(x + i + 0.5) / (x + i + 1) / (x + i);
			ret -= Math.cos(-x + i + 0.5) / (-x + i + 1) / (-x + i);
		}
		ret *= 0.5 / Math.sin(0.5);
		ret += sinc(x);
		return a * ret;
	}

	void run() {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		double ans = 0;
		for (int i = 0; i < n; ++i) {
			double x = sc.nextDouble();
			double a = sc.nextDouble();
			ans += solve(x, a);
		}
		System.out.println(ans);
	}

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