
typedef long long ll; 
const ll mod = 1e9+7;
#define fo(i, n) for (int i = 0; i < n; i++)
#define sc(n) scanf("%d", &n) 
using namespace std;

int n;

void solve() {
    cin >> n;
    printf("%.18f", 2*3.14159265389793238*n);
}

int main() {
    int t = 1;
    //scanf("%d", &t);
    while (t--) {
        solve();
    }
    return 0;
}