#include <bits/stdc++.h>
using namespace std;
using lli = long long int;
using ulli = unsigned long long int;
using ld = long double;
const lli MOD = (int)1e9 + 7;
const lli MOD1 = (int)1e9 + 9;
const lli MOD2 = 998244353;
const ld pi = atan2(0, -1);
const ld eps = 1e-9;
lli tests = 1;
template <class x, class y>
x max(x a, y b) {
  return (a > b) ? a : b;
}
template <class x, class y>
x min(x a, y b) {
  return (a < b) ? a : b;
}
template <class x, class y>
x log(x n, y r) {
  return (n > r - 1) ? 1 + log(n / r, r) : 0;
}
int32_t main() {
  auto start = clock();
  ios_base::sync_with_stdio(0);
  cout.tie(0);
  cin.tie(0);
  cout << fixed << setprecision(20);
  for (lli test = 1; test <= tests; test++) {
    lli n;
    cin >> n;
    cout << double(2 * n * pi);
  }
  cerr << endl
       << endl
       << endl
       << "Time : " << double(clock() - start) / CLOCKS_PER_SEC << endl;
  return 0;
}
