#include <bits/stdc++.h>
using namespace std;
using ll = long long;
const int mod = 1e9 + 7;
ll pwr(ll a, ll b);
int main() {
  cin.sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  int r;
  cin >> r;
  cout << fixed << setprecision(6) << (2.0 * 3.14 * r) << "\n";
}
ll pwr(ll a, ll b) {
  a %= mod;
  ll res = 1;
  while (b > 0) {
    if (b & 1) res = res * a % mod;
    a = a * a % mod;
    b >>= 1;
  }
  return res;
}
