#include <bits/stdc++.h>
#pragma GCC optimize("O3")
#pragma GCC optimize("unroll-loops")
const long long INF = 0x3f3f3f3f3f3f3f3f;
const long long llinf = (1LL << 62);
const int inf = (1 << 30);
const int nmax = 1e5 + 50;
const long long mod = 1e9 + 7;
using namespace std;
long double r;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(0);
  cerr.tie(0);
  cout.tie(0);
  cin >> r;
  cout << fixed << setprecision(10) << 2 * acos(-1) * r << endl;
  return 0;
}
