#include <bits/stdc++.h>

using namespace std;
typedef pair <int, int> pii;
#define pb push_back
#define mp make_pair
#define ll long long
#define PI acos(-1)

int main() {
    int r;
    cin >> r;
    cout << fixed << setprecision(2) << 2 * PI * r ;
    return 0;
}
