#include <algorithm>
#include <array>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <vector>
using namespace std;
#define int long long

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

    cout << fixed << setprecision(16);

    int r;
    cin >> r;

    cout << 2 * M_PI * r << endl;
}
