#include <bits/stdc++.h>
using namespace std;
const long long INF = 100000000000000001;
struct edge {
  int to, cost;
};
struct pos {
  int x, y, cost;
};
int dy[] = {0, 0, 1, -1};
int dx[] = {1, -1, 0, 0};
struct Road {
  double cost;
  int a, b;
};
int main() {
  int r;
  cin >> r;
  cout << r * 2 * 3.1415 << endl;
  return 0;
}
