#include <iostream>
#include <algorithm>
#include <vector>
#define PI 3.14159265358979323846
using namespace std;
using ll = long long;
using P = pair<int,int>;

int main() {
int r;
cin >> r;
double l;
l = 2 *r * PI;
    cout << fixed << setprecision(20);
    cout << l << endl;

return 0;

}
