#include <bits/stdc++.h>
#pragma GCC optimize("O3")
using namespace std;
const long long INF = 1000000000000000000LL;
long long power(long long a, long long b) {
  long long i = 1;
  while (b) {
    if (b % 2) i = (i * a) % 1000000007;
    b /= 2;
    a = (a * a) % 1000000007;
  }
  return i;
}
long long mulmod(long long a, long long b) {
  long long res = 0;
  a = a % 1000000007;
  while (b > 0) {
    if (b % 2 == 1) res = (res + a) % 1000000007;
    a = (a * 2) % 1000000007;
    b /= 2;
  }
  return res % 1000000007;
}
const int N = 1e5 + 5;
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(NULL);
  cout.tie(NULL);
  cout << fixed;
  cout.precision(10);
  int b;
  cin >> b;
  double ans = b * 2 * 3.14;
  cout << ans;
  return 0;
}
