#include <iostream>
#include <cmath>
#include <iostream>
#include <iomanip>

using namespace std;
int main(void){
    // Your code here!
    
    long long R;
    const double PI=3.14159265358979323846;
    
    cin >> R;
    
    cout << setprecision(15) << 2* double(R)*PI<<endl;
    return 0;
}
