#include <bits/stdc++.h>
using namespace std;
const long long N = 1e5 + 5, mod = 1e9 + 7, mod1 = 998244353, mod2 = 1e9 + 9,
                inf = 1e18 + 7;
const long long infll = 1e18 + 7;
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long n;
  cin >> n;
  n *= 2;
  long double ans = 3.14159265358 * n;
  cout << fixed << setprecision(6) << ans;
}
