#include <iostream>
#include <iomanip>
#include <string>
#include <vector>
#include <queue>
#include <algorithm>
#include <utility>
#include <unordered_map>
#include <cmath>
using namespace std;

typedef unsigned int uint;
typedef long long llong;
typedef unsigned long long ullong;
// -2.1e-9 <= int <= 2.1e9
// -9.2e-18 <= long long <= 9.2e18
// 0 <= unsigned int <= 4.2e9
// 0 <= unsigned long long <= 18.4e18

const double PI=3.1415926535897932;
int main()
{
    int R;
    cin >> R;
    double ans = 2.0 * PI * R;
    cout << fixed << setprecision(14);
    cout << ans << endl;
}
