#include <bits/stdc++.h>
using namespace std;
const long long int MAX_N = 1 << 17;
using namespace std;
long long int dy[] = {0, 0, 1, -1, 0};
long long int dx[] = {1, -1, 0, 0, 0};
template <class T>
bool chmax(T& a, const T& b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <class T>
bool chmin(T& a, const T& b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
long long int gcd(long long int a, long long int b) {
  return b ? gcd(b, a % b) : a;
}
struct aaa {
  aaa() {
    cin.tie(0);
    ios::sync_with_stdio(0);
    cout << fixed << setprecision(20);
  };
} aaaaaaa;
signed main() {
  long long int r;
  cin >> r;
  cout << (long double)r * 2 * M_PI << endl;
  return 0;
}
