#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef pair<int, int> pii;

const int N = 1e5 + 11;
const int M = 1e6 + 11;
const int MOD = 1e9 + 7;
const double PI = acos(-1.0);

int main(int argc, char **args){
    freopen("in.txt", "r", stdin);
    double r; cin >>r;
    cout << 2 * PI * r<<"\n";
return 0;
}