#include <bits/stdc++.h>
using namespace std;
const long long int MOD = 1e9 + 7;
const long long int INF = 1e18;
const long long int N = 1e6 + 20;
vector<long long int> g[N];
vector<long long int> vis(N);
int32_t main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long int tc = 1;
  while (tc--) {
    const double pi = 3.14;
    double r;
    cin >> r;
    cout << 2 * pi * r << '\n';
  }
}
