#include <iostream>
#include <string>
using namespace std;
 
int main() {
    int R;
    cin >> R;
 
    cout << 2*R*3.14159265 << endl;
}
#include <iostream>
#include <string>
using namespace std;

int main() {
    int R;
    cin >> R;

    cout << 2*R*3.14159265 << endl;
}