#include <bits/stdc++.h>

using namespace std;
void fast_io() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
}

int main() {
    fast_io();
    int n;
    cin >> n;
    long double ans = 0;
    for (int i = 0; i < n; i++) {
        long double x, a;
        cin >> x >> a;
        ans += a;
    }
    ans *= M_PI;
    cout << fixed << setprecision(16) << ans << endl;
}