double get_prob(double x, double n) {
    return exp(-x / n) / sqrt(pi * n);
}

int main() {
    int n; // 16
    cin >> n;
    vector<pair<double, double> > P(n);
    for (int i = 0; i < n; i++) {
        cin >> P[i].first >> P[i].second;
    }

    const double MAX_CORD = 10;
    const double STEP = 0.3;
    const double N0 = 1; // sigma
    double P_X = 1.0 / n;
    double sum = 0;

    for (double x = -MAX_CORD; x <= MAX_CORD; x += STEP) {
        for (double y = -MAX_CORD; y <= MAX_CORD; y += STEP) {
            if (x * x + y * y > 0.01) {
                double p_y = 0;
                for (int i = 0; i < n; i++) {
                    double dx = x - P[i].first;
                    double dy = y - P[i].second;
                    double p_y_xi = get_prob(dx * dx + dy * dy, N0);
                    p_y += p_y_xi * P_X;
                }
                for (int i = 0; i < n; i++) {
                    double dx = x - P[i].first;
                    double dy = y - P[i].second;
                    double p_y_x = get_prob(dx * dx + dy * dy, N0);
                    double f = p_y_x * P_X * log(p_y_x / p_y);
                    sum += f * STEP * STEP;
                }
            }
        }
    }
    printf("%0.10f", sum);
}