#include <bits/stdc++.h>
using namespace std;
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  long long n, i, j, count = 0, ans = 0, l, r, x, y, sum = 0, mx = 0;
  cin >> n;
  n = n * 2;
  double k = 3.1415;
  k = k * double(n);
  cout << k;
  return 0;
}
