#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<ll, ll>;
template <class T>
using V = vector<T>;
constexpr ll INF = (1LL << 30) - 1LL;
constexpr ll MOD = 998244353LL;
constexpr int dx[4] = {0, 1, 0, -1}, dy[4] = {1, 0, -1, 0};
template <class T>
void chmin(T &a, T b) {
  a = min(a, b);
}
template <class T>
void chmax(T &a, T b) {
  a = max(a, b);
}
void debug() { cerr << "ok" << endl; }
template <class T>
void printv(const vector<T> &v) {
  for (int i = 0; i < v.size(); i++)
    cout << v[i] << (i + 1 == v.size() ? '\n' : ' ');
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  double r;
  cin >> r;
  cout << 2 * r * M_PI << '\n';
}
