#include <iostream>
#include <string>
#include <algorithm>
#include <math.h>
#include <vector>
#include <iomanip>

using namespace std;

#define fori(x) for (int i = 0; i < x; ++i)
#define forj(x) for (int j = 0; j < x; ++j)

int main() {
    int r;
	cin >> r;
    cout<<fixed<<setprecision(5)<<((float)r*2)*3.14159265;
	return 0;
}