#include <bits/stdc++.h>
using namespace std;
long long int powr(long long a, long long b) {
  if (b == 0) {
    return 1;
  }
  if (b % 2 == 0) {
    return powr(((a % 1000000007) * (a % 1000000007)) % 1000000007, b / 2);
  } else {
    if (b == 1) {
      return a;
    } else {
      return (a * powr(((a % 1000000007) * (a % 1000000007) % 1000000007),
                       (b - 1) / 2)) %
             1000000007;
    }
  }
}
int gcd1(int a, int b) {
  if (a == 0)
    return b;
  else {
    return gcd1(b % a, a);
  }
}
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int r;
  cin >> r;
  cout << fixed << setprecision(4) << 2 * 3.14159 * (float(r));
  return 0;
  cerr << "Time : " << 1000 * ((double)clock()) / (double)CLOCKS_PER_SEC
       << "ms\n";
}
