#include <bits/stdc++.h>
using namespace std;
using ll = long long;
ll MOD = 1000000007;
ll mod = 998244353;
int inf = 1000001000;
ll INF = 1e18 + 5;
int main() {
  int r;
  cin >> r;
  cout << 3.14159 * r << endl;
}
