#include <bits/stdc++.h>
using namespace std;
const int MOD = 1000000007;
const int INF = 2147483647;
int main(void) {
  long double r;
  cin >> r;
  long double ans = r * 6.28318530717958623200;
  printf("%.12Lf\n", ans);
  return 0;
}
