#include<iostream>
#include<string>
#include<vector>
#include<iomanip>
#include<algorithm>
#include<queue>
#include<stack>
#include<list>
#include<map>
#include<deque>
#include<math.h>
using namespace std;
#define ll long long
int main(){
    double R;
    cin >> R;
    cout << setprecision(20) << 2.0*R*3.14159265358979323846264;
    return 0;
}
