#include <bits/stdc++.h>
using namespace std;
using in = int64_t;
const double PI = 3.14159265358979323846;
const in MOD = 1000000007;
const in INF = 1e18 + 7;
const int inf = 1e9 + 7;
const vector<int> dx = {1, 0, -1, 0};
const vector<int> dy = {0, 1, 0, -1};
int main() {
  int n;
  cin >> n;
  printf("%.4f", n * PI);
}
