#include <iostream>
#include <iomanip>
using namespace std;
int main() {
    int n; cin >> n;
    double ans = 0;
    double x, a;
    for (int i = n; i--;) {
        cin >> x >> a;
        ans += a;
    }
    cout << fixed << setprecision(20) << ans * 3.14159265358979323846264338327950288 << "\n";
}