#define _USE_MATH_DEFINES
#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;

#define FOR(i,a,b) for (int i = (a); i < (b); i++)
#define F0R(i,b) FOR(i,0,b)
#define RFO(i,a,b) for (int i = (b-1); i >= a; i--)
#define RF0(i,b) RFO(i,0,b)
#define F first
#define S second

int main() {
  cin.tie(0);
  ios_base::sync_with_stdio(false);


  int R;
  cin >> R;
  R *= 2;

  cout << ((long double)M_PI * R);
}
