#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define all(v) (v).begin(),(v).end()
#define out(a) cout << (a) << endl
using namespace std;
using ll = long long;
int main(){
    int r;
    cin >> r;
    cout << fixed << setprecision(20);
    out(r*2*M_PI);
}