#include<cstdio>
#include<cmath>
#include<cstring>
#include<string>
#include<algorithm>
#include<queue>
#include<map>
#include<set>
#include<vector>
#include<functional>
#include <random>
#include<iostream>
#include<iomanip>
using namespace std;


int main() {
	int R;
	cin >> R;
	cout << (double)R * 2 * 3.14159265358979;
}