#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
using ll = long long;
ll GetDigit(ll num){ return log10(num)+1; } //numの桁数を求める
#define pi 3.141592653589793238462643383279 
int main()
{
    int R;
    cin >> R;
    cout << 2*R*pi << endl;
}