#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long ans, sum, tot, cnt, mx, mn, idx, chk, tmp, mid, lft, rgt, pos;
  long long tc, n, m, a, b, c, d, g, h, l, r, x, y, i, j, k;
  cin >> a;
  double rr = a;
  cout << fixed << setprecision(20) << 2.0 * 2 * acos(0.0) * rr << '\n';
  return 0;
}
