#include <bits/stdc++.h>
using namespace std;
int64_t a, b, c, d, e, f, g, h, i, j, k, l, m, n, o, p, q, r, s, t, u, v, w, x,
    y, z;
bool flagdebug = false;
int main() {
  cin >> r;
  cout << r * 2 * 3.14159 << endl;
  return 0;
}
