import java.util.*;
import java.lang.*;
import java.io.*;

class A278 {
	public static void main (String[] args) throws java.lang.Exception {
		Scanner scanner = new Scanner(System.in);
        double numerator = 0;
		int denominator = 0;
		while (scanner.hasNext()) {
			int n = scanner.nextInt();
			double x = scanner.nextDouble();
			numerator += n*x;
			denominator += n;
		}
		double result = numerator / denominator;
		System.out.println(result);
	}
}