#include <bits/stdc++.h>
using namespace std;
const long long N = 3e6 + 7, M = 1000000007, inf = INT_MAX;
const long long INF = LLONG_MAX;
long long n, Q;
long long arr[N + 2];
long double pi = 3.14159265;
void solve() {
  double n;
  cin >> n;
  cout << fixed << setprecision(12) << 2 * pi * n << '\n';
}
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cout << setprecision(9) << fixed;
  clock_t z = clock();
  long long test = 1;
  while (test--) solve();
}
