#include<iostream>
#include<cstdio>
#include<string>
#include<vector>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<queue>
#include<stack>
#include<set>
#include<map>
#define ll long long
#define PI acos(-1.0)
#define MAX 0x3fffffff
using namespace std;

double r;

int main(){
	ios::sync_with_stdio(false);
	cin.tie(0);
	cin >> r;
	printf("%.8lf\n", 2 * PI*r);
	return 0;
}