#include <iostream>
#include <cmath>
#include <algorithm>
#include <string>
#include <vector>
#include <math.h>

typedef unsigned int u32;
typedef int i32;
typedef unsigned long long int u64;
typedef long long int i64;
typedef uint16_t u16;
typedef int16_t i16;
typedef uint8_t u8;
typedef int8_t i8;

const double PI = 3.141592653589793238463;

using namespace std;

const int MAXN = 1000500;
// const int MAXN = 1050;
// const long long int mod = 1000003;
//const int MAXVALUE = 1000000500;
//const long double EPS = 1e-7;

i64 mod = 1000000009;
i64 T, B;

i64 s_left = -1;
i64 d_left = -1;

i64 n, m, r;

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

	cout.precision(16);

	cin >> r;
	cout << 2 * r*PI << endl;


	//a.resize(n + 1);
	//dp.assign(n + 1, -1);	

	return 0;
}