#include <bits/stdc++.h>
using namespace std;
#pragma GCC target("avx2")
#pragma GCC optimization("unroll-loops")
#pragma GCC optimize("O2")
constexpr long long int dx[] = {-1, 0, 1, 0, 1, 1, -1, -1};
constexpr long long int dy[] = {0, -1, 0, 1, 1, -1, 1, -1};
constexpr long long ll INF = 1999999999999999997;
constexpr long long int inf = INT_MAX;
constexpr long long int MAXSIZE = long long int(1e6) + 5;
constexpr auto PI = 3.14159265358979323846L;
constexpr auto oo = numeric_limits<long long int>::max() / 2 - 2;
constexpr auto eps = 1e-6;
constexpr auto mod = 1000000007;
constexpr auto MOD = 1000000007;
constexpr auto MOD9 = 1000000009;
constexpr auto maxn = 100006;
void fastio() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
}
int32_t main() {
  fastio();
  double r;
  cin >> r;
  setprecision(20);
  cout << 2 * PI * r;
}
