#include <bits/stdc++.h>
using namespace std;
struct Fast {
  Fast() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(20);
  }
} fast;
long long mod = 1000000007;
long long modpow(long long m, long long n) {
  if (n == 0) return 1;
  if (n % 2 == 0) {
    long long t = modpow(m, n / 2);
    return (t * t) % mod;
  } else {
    return (modpow(m, n - 1) * m) % mod;
  }
}
int power(long long x, long long y, long long p) {
  long long res = 1;
  x = x % p;
  while (y > 0) {
    if (y & 1) res = (res * x) % p;
    y = y >> 1;
    x = (x * x) % p;
  }
  return res;
}
long long modInverse(long long n, long long p) { return power(n, p - 2, p); }
long long nCrModPFermat(long long n, long long r, long long p) {
  if (r == 0) return 1;
  long long fac[n + 1];
  fac[0] = 1;
  for (long long i = 1; i <= n; i++) fac[i] = fac[i - 1] * i % p;
  return (fac[n] * modInverse(fac[r], p) % p * modInverse(fac[n - r], p) % p) %
         p;
}
void yes() {
  cout << "Yes" << endl;
  exit(0);
}
void no() {
  cout << "No" << endl;
  exit(0);
}
signed main() {
  long long n, a, b;
  cin >> a;
  double pi = 3.141592653589793238;
  cout << 2 * a * pi << endl;
}
