#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using vi = vector<int>;
using vll = vector<long long>;

int dx[4] = {1, 0, -1, 0};
int dy[4] = {0, 1, 0, -1};

constexpr long long mod = 1000000007;

#define rep(i, n) for (int i = 0; i < n; i++)

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

    double r;
    cin >> r;

    r = 2 * 3.1415926535 * r;

    printf("%.4lf\n", r);
}