#include <iostream>
#include <vector>
using namespace std;
 
int main() {
	vector<int> n;
	vector<double> x;
	int a, sum2 = 0;
	double b, res = 0, sum1 = 0; 
	while (cin >> a >> b) 
	{ 
		n.push_back(a); 
		x.push_back(b); 
	} 
	for (int i=0; i < x.size(); i++) 
	{ 
		sum1 += n[i] * x[i];
		sum2 += n[i];
		res = sum1 / sum2;
	} 
	cout << res << endl; 
	return 0;
}