#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define rep1(i, n) for (int i = 1; i <= (int)(n); i++)
typedef int64_t Int;


int main(){
    double R; cin >> R;

    cout << setprecision(10);
    cout << 2 * R * 3.141592 << endl;
}