#include <bits/stdc++.h>
using namespace std;
const long long N = 2e5 + 5, inf = 1e18, mod = 1e9 + 7;
int32_t main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long r;
  cin >> r;
  cout << fixed << setprecision(12);
  double ans = 2 * (3.14159 * r);
  cout << ans;
}
