#include <bits/stdc++.h>
#define rep(i, n) for(int i = 0; i < n; i++)

using namespace std;
using ll = long long;


int main(){
    double r; cin >> r;
    double pi = 3.141592;
    double ans = 2* r * pi;
    cout << ans << endl;

    return 0;
}

