#include <bits/stdc++.h>
using namespace std;
template <typename A, typename B>
bool cmin(A &a, const B &b) {
  return a > b ? (a = b, true) : false;
}
template <typename A, typename B>
bool cmax(A &a, const B &b) {
  return a < b ? (a = b, true) : false;
}
const double PI = acos(-1);
const double EPS = 1e-9;
long long inf = sizeof(long long) == sizeof(long long) ? 2e18 : 1e9 + 10;
long long dx[] = {-1, 0, 1, 0};
long long dy[] = {0, 1, 0, -1};
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  double a;
  cin >> a;
  cout << fixed << setprecision(9) << a * PI * 2 << endl;
}
