import java.util.*;
import java.math.*;

public class Main {
    static final int MOD = 1_000_000_007; // 10^9+7
    // static final int MAX = 2_147_483_646; // intの最大値
    static final int INF = 1_000_000_000; // 10^9
    static final int MAX = 10_000_000;

    public static void main(String args[]) {
        Scanner sc = new Scanner(System.in);
        double n = sc.nextDouble();
        System.out.println(2*n*Math.PI);
        

    }

}
