#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 5;
long double EPS = 1e-9;
inline long long add(long long x, long long y) {
  x += y;
  if (x >= 1000000007) x -= 1000000007;
  return x;
}
inline long long sub(long long a, long long b) {
  if (a < b) swap(a, b);
  return (((a % 1000000007) - (b % 1000000007) + 1000000007) % 1000000007);
}
inline long long mul(long long x, long long y) {
  return (x % 1000000007 * 1ll * y % 1000000007) % 1000000007;
}
inline long long pwr(long long a, long long b) {
  long long x = 1 % 1000000007;
  while (b) {
    if (b & 1) x = mul(x, a);
    a = mul(a, a);
    b >>= 1;
  }
  return x % 1000000007;
}
inline long long inv(long long a) {
  return pwr(a, 1000000007 - 2) % 1000000007;
}
long long gcd(long long a, long long b) {
  if (!a) return b % 1000000007;
  return gcd(b % a, a) % 1000000007;
}
template <typename Arg1>
void __f(const char* name, Arg1&& arg1) {
  cout << name << " : " << arg1 << "\n";
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args) {
  const char* comma = strchr(names + 1, ',');
  cout.write(names, comma - names) << " : " << arg1 << " | ";
  __f(comma + 1, args...);
}
void pre() {}
void solve() {
  long long n;
  cin >> n;
  cout << 2 * acos(-1.0) * n << "\n";
}
signed main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  cout << fixed << setprecision(15);
  long long tc;
  tc = 1;
  while (tc--) {
    solve();
  }
}
