#include <bits/stdc++.h>

using namespace std;

#define M (ll)(1e9+7)
#define EPS (double)(1e-12)
#define ll long long
#define N (ll)(2*1e5)



int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    double res;
    int value;

    cin >> value;

    res = (double)2*value*(double)M_PI;

    cout << setprecision(10) << fixed << res << endl;

    return 0;
}