#include <bits/stdc++.h>
using namespace std;
const int N = 2e5 + 10, mod = 1e9 + 7;
int main() {
  int tc = 1;
  while (tc--) {
    int r;
    cin >> r;
    printf("%.20lf\n", 2 * acos(-1) * r);
  }
  return 0;
}
