#include <bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < (n); ++i)
using namespace std;
using ll = long long;
using P = pair<int,int>;
 
int main(){
    double n;
    const double PI = (acos(-1));
    cin >> n;
    cout << fixed << setprecision(10) << 2*n*PI << endl;
    return 0;
}