#include <bits/stdc++.h>
using namespace std;
const long double PI = acos(-1);
long long powm(long long a, long long b) {
  long long res = 1;
  while (b) {
    if (b & 1) res = (res * a) % 1000000000;
    a = (a * a) % 1000000000;
    b >>= 1;
  }
  return res;
}
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  return gcd(b, a % b);
}
const long long maxn = 2 * 1e5 + 10;
void solve() {
  long long r;
  cin >> r;
  double ans = 2.0 * (double)3.1415926535897932384626433832 * (double)r;
  cout << fixed << setprecision(20) << ans << endl;
}
signed main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  ;
  long long t = 1;
  for (long long i = 1; i <= t; i++) {
    solve();
  }
  return 0;
}
