#include <bits/stdc++.h>
using namespace std;
const long long inf = 1e12 + 10;
const long long maxn = 2e5 + 10;
const long long mod = 1e9 + 7;
bool mark[maxn];
vector<long long> adj[maxn];
int32_t main() {
  ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
  long long r;
  cin >> r;
  double p = 3.14;
  cout << r * 2 * p << endl;
  return 0;
}
