//2008C Scott
#include <iostream>
#include <iomanip>
#include <vector>

using namespace std;

int main() {
	int n;
	double v, time;
	double sum = 0;
	vector<double> speeds;
	while (cin >> n && n != 0) {
		speeds.clear();
		sum = 0;
		for (int i = 0; i < n; i++) {
			cin >> v >> time;
			for (int j = 0; j < time; j++) {
				speeds.push_back(v);
			}
		}
		
		for (int i = 0; i < speeds.size(); i++) {
			sum += speeds[i];
		}
		cout << fixed << setprecision(2) << double(sum)/double(speeds.size()) << endl;
	}

	return 0;
}