#include <bits/stdc++.h>
using namespace std;
using u8 = uint8_t;
using u16 = uint16_t;
using u32 = uint32_t;
using u64 = uint64_t;
using i64 = int64_t;
int main() {
  cin.tie(0);
  ios_base::sync_with_stdio(false);
  int R;
  cin >> R;
  R *= 2;
  cout << ((long double)M_PI * R);
}
