#include <bits/stdc++.h>
#define S second
#define F first
#define ll long long
#define ld long double

using namespace std;

const ll N = 2e5 + 5, M = N * 2;

int main()
{
    ll r;
    scanf("%lld", &r);
    ld ans = 2 * acos(-1) * r;
    __mingw_printf("%.3Lf\n", ans);
}
