#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < n; i++)
#define MOD 1000000007;
#define MOD2 998244353;
#define INF ((1<<30)-1);
#define LINF (1LL<<60);
#define EPS (1e-10);
typedef long long Int;
typedef pair<Int, Int> P;

#define PI 3.141592653589793

int main() {
    double n;
    cin >> n;
    printf("%.10lf\n", 2*n*PI);
}