#include <iostream>
#include <vector>
#include <algorithm>
#define rep(i, n) for(int i = 0; i < (int)(n); ++i)
#define REP(i, n) for(int i = 1; i < (int)(n); ++i)
using namespace std;

int main() {
    double pi = 3.1415926535;
    double R;
    cin >> R;
    cout << 2 * R * pi << endl;
    return 0;
}
