#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 FORR(i, m, n) for (int i = m; i >= n; i--)
#define SORT(v, n) sort(v, v + n);
#define VSORT(v) sort(v.begin(), v.end());
#define ll long long
#define ull unsigned long long
#define pb(a) push_back(a)
#define INF 100000000001
using namespace std;
typedef pair<int, int> P;
typedef pair<ll, ll> LP;
typedef pair<int, P> PP;
typedef pair<ll, LP> LPP;

int dy[] = {0, 0, 1, -1, 0};
int dx[] = {1, -1, 0, 0, 0};


int main()
{
    double r;
    cin >> r;
    cout << r * 2.0 * 3.14 << endl;
}
