#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const ll INF = ll(1e12);
const ll mod = ll(1e9) + 7;
ll gcd(ll a, ll b) {
  if (a < b) swap(a, b);
  if (b == 0) return a;
  return gcd(b, a % b);
}
ll lcm(ll a, ll b) {
  ll g = gcd(a, b);
  return a / g * b;
}
ll factorial(ll n) {
  ll t = 1;
  for (ll(i) = 1; (i) <= (n); ++(i)) {
    t *= i;
    t %= mod;
  }
  return t;
}
ll power(ll a, ll b) {
  a %= mod;
  if (b == 0) {
    return 1;
  }
  ll ans = power(a, b / 2);
  ans = ans * ans % mod;
  if (b % 2 == 1) {
    ans = ans * a % mod;
  }
  return ans;
}
ll combination(ll a, ll b) {
  if ((a == b) || (b == 0)) {
    return 1;
  }
  if (a < b) return 0;
  ll ans = 1;
  for (ll i = 0; i < b; i++) {
    ans *= (a - i);
    ans /= (i + 1);
    ans %= mod;
  }
  return ans;
}
int main() {
  double r;
  cin >> r;
  printf("%.9f", r * 3.141592);
  return 0;
}
