#include <bits/stdc++.h>
#define REP(i, n) for(int i = 0; i < n; i++)
#define REPR(i, n) for(int i = n; i >= 0; i--)
#define FOR(i, m, n) for (int i = m; i < n; i++)
#define INF 2e9
#define ALL(v) v.begin(), v.end()
using namespace std;
typedef long long ll;
typedef vector<ll> vll;
typedef vector<vll> vvll;


int main()
{
    double r; cin >> r;
    cout << r * 2 * M_PI << endl;
    return 0;
}