#include <bits/stdc++.h>
#define rep(i,n) for (int i = 0; i < (n); ++i)
using namespace std;
using ll = long long;
typedef std::pair<int,int> ipair;

int main()
{
    float r;
    cin >> r;
    float len;
    const double PI = acos(-1);
    len = 2*r*PI;
    
    cout << len << endl;

    return 0;
}