#include<iostream>
#include<algorithm>
#include<vector>
#include<string>
#include<cmath>
#include<map>
#include<iomanip>
#define rip(i,n) for(int i=0;i<n;i++)
#define ll long long
#define lb long double
#define vec vector<int>
using namespace std;
int main() {
	int a;
	cin >> a;
	cout << 2 * a * 3.14159265 << endl;
}
