#include <bits/stdc++.h>
using namespace std;
const long long MAX = 1e6 + 6;
const long long inf = 1e9;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long tc, n, m, a, b;
  tc = 1;
  while (tc--) {
    cin >> n;
    long double ans = 6.28318530717958623200 * 0.5 * (1.0 * n);
    cout << setprecision(10) << ans << endl;
  }
}
