#include <bits/stdc++.h>
#pragma GCC optimize(2)
#pragma GCC optimize(3)
#pragma GCC optimize("Ofast")
const int INF = 0x3f3f3f3f;
const int NINF = -INF - 1;
const double PI = acos(-1);
const int mod = 1e9 + 7;
const int maxn = 5e5 + 50;
using namespace std;
int main() {
  int n;
  cin >> n;
  printf("%.5lf", PI * 2 * n);
  return 0;
}
