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

int main() {
    float r;
    cin >> r;
    float ans;
    ans = r * 3.1415926 * 2.0;
    printf("%.3f", ans);
    return 0;
}