#include <bits/stdc++.h>
using namespace std;
const long long INF = 10e16;
const int maxN = 200010;
const int maxD = 15;
const int maxH = 100;
const int MOD = 1000000007;
vector<string> ans;
vector<int> p;
int main() {
  double p = 3.1415926;
  int R;
  cin >> R;
  double ans = p * R * 2;
  printf("%f\n", ans);
  return 0;
}
