#include <bits/stdc++.h>

#define rep(i, n) for(int i = 0; i<(n); i++)
#define chmax(x, y) x = max(x, y)
#define chmin(x, y) x = min(x, y)
using namespace std;
using ll = long long;

const double PI = 3.14159265358979323846;

int main() {
    double R;
    cin >> R;
    double ans = 2 * PI * R;
    printf("%.12f\n", ans);
}
