#include <iostream>
#include <iomanip>
#include <cmath>

using namespace std;

int n;

double a, b, sum1 = 0.0, sum2 = 0.0;

int main() {
    cin >> n;
    while(n-- > 0) {
        cin >> a >> b;
        sum1 += a * b;
        sum2 += b;
    }
    cout << fixed << setprecision(1) << round(sum1 / sum2 * 10.0) / 10.0;
    return 0;
}