#include <bits/stdc++.h>
#define be(v) (v).begin(),(v).end()
#define pb(q) push_back(q)
#define rep(i, n) for(int i=0;i<n;i++)
#define all(i, v) for(auto& i : v)
typedef long long ll;
using namespace std;
const ll mod=1000000007, INF=(1LL<<60);
#define doublecout(a) cout<<fixed<<setprecision(10)<<a<<endl;

int main() {
    cin.tie(0);
    cout.tie(0);
    ios::sync_with_stdio(false); 
    int n;
    cin >> n;
    double ans = 0.0, x, a;
    rep(i, n) {
        cin >> x >> a;
        ans += a;
    }
    doublecout(ans * M_PI);
    return 0; 
}