#include<bits/stdc++.h>
#define rep(i, n) for (ll i = 0; i < (ll)(n); ++i)
#define repk(i,k,n) for (ll i = k; i < (ll)(n); ++i)
#define MAX 500000001000000000
#define MOD 1000000007
typedef long long  ll;

using namespace std;

double ans, cnt;

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

	int N;
	cin >> N;

	const double PI = 3.14159265358979323846;


	ans = 2 * N * PI;

	printf("%.11lf", ans);

	return 0;
}