#include <bits/stdc++.h>
using namespace std;
const long long INF = 1000000000000000000;
const long long mod = 1000000007;
long long test[345678], aa[345678][2];
signed main() {
  long long n, m, cnt = 0, ans = 0;
  string s, t;
  cin >> n;
  printf("%f\n", n * 2 * 3.141592);
  return 0;
}
